1. Male rats of approximately 120 g body-weight were maintained on a commercial stock diet containing 204 g crude protein (nitrogen × 6.25)/kg, a hydroxyproline-free high-protein (HP) diet containing 200 g casein/kg as the only protein source, or a low-protein (LP) diet containing 40 g casein/kg. After 6 weeks on these diets half of each group was transferred to a non-protein (NP) diet and the experiment was continued for a further 6 weeks. Animals from each group were killed at 4 d, 3 weeks and 6 weeks after the transfer to the NP diet.

2. Throughout the experiment the urinary excretion of N, hydroxyproline and creatinine, and the content and solubility of the skin collagen were determined.

3. When compared with a control group killed at the beginning of the experiment the rats maintained on the LP diet showed an increase of 25% in total N content of the skin but collagen content increased by 100%. Rats transferred from the HP to the NP diet lost both N and collagen from the skin, but those transferred from the LP to the NP diet lost N but increased the collagen content by 42%.

4. Protein deprivation brought about marked changes in the solubility of the skin collagen, suggesting an increase in the rate of maturation of skin collagen.

1. Semi-synthetic diets, with dried microbial cells (Pruteen) as the nitrogen source, were used to measure N retention in 50 kg pigs which had been given only sufficient N (5.3 g/d) to maintain N equilibrium for the previous 12 d. Control pigs were given 33.2 g N/d.

2. Metabolic faecal N losses were 1.62 g/d (1.2 g/kg dry matter eaten) and endogenous urinary losses were 3.90 g/d.

3. Realimentation of N-deprived pigs was achieved with diets providing 20.0, 33.2 and 67.4 g N/d and daily rates for N retention were 11.2, 17.8 and 25.9 g respectively; the corresponding value for control pigs was 15.0 g. ‘Catch-up’ protein growth was demonstrated in pigs given both 33.2 and 67.4 g N/d. In the former instance, this was associated with an increase in the efficiency of utilization of dietary N.

4. The biological value of the protein in Pruteen was 0.85, and it appeared that under conditions of increased demand for N the pig could utilize some of the nucleic acid-N fraction of this protein source.

1. Male and female rats aged 3 weeks were divided into two groups. One group of each sex was allowed unlimited access to the stock diet, the other group was given the stock diet in restricted amounts for 10 weeks so that the males gained only 19 g and the females 21 g in comparison with 176 g and 116 g for the well-nourished males and females respectively. The undernourished animals were then rehabilitated by being allowed the stock diet ad lib.

2. Five animals of each sex were killed at various stages of the experiment, their bodies analysed for fat and nitrogen, and the size and number of fat cells determined in specific fat depots.

3. The undernourished rats failed to make a complete recovery and were significantly smaller than the controls of the same sex at 172 d of age when the experiment terminated.

4. The previously undernourished rats deposited significantly more fat in their bodies during rehabilitation than the control animals in the same number of days and over the same gain in body-weight.

5. There were no significant differences in the number of cells containing fat at the abdominal fat site between the undernourished and rehabilitated animals and the controls at any stage, nor were there any significant differences in apparent fat cell numbers between the control and rehabilitated animals at any of the other sites studied when the experiment ended at 172 d.

1. A combination of a mineral balance and a radioactive technique has been used to study the effect of the dietary calcium intake of ewes in pregnancy on their Ca and phosphorus metabolism in lactation.

2. Ewes which had received a Ca-deficient diet in pregnancy absorbed Ca at a greater rate and with a greater efficiency in early lactation than did the control animals, which had received an adequate Ca intake in pregnancy. The apparent absorption of P was also higher in the Ca-deficient sheep.

3. Since both groups of sheep received the same high Ca intake in lactation, the increased rate of absorption of Ca must have resulted from an increased rate of active transport rather than an increased rate of diffusion.

4. Retention of both Ca and P was positive in the Ca-deficient ewes but negative in the control ewes. This difference in retention reflected a difference in bone metabolism.

5. The maximum rate of absorption possible, which is equal to the rate of irreversible loss of Ca from the rapidly exchangeable pool, was much greater in the Ca-deficient lactating ewes than was previously found in Ca-deficient wethers. This difference is due to the additional pathway of loss of Ca in the milk of the lactating animals.

6. The relationship between these findings and the prevention of milk fever in dairy cows by manipulation of the dietary Ca intake is discussed.

1. Five sheep were given a meal while they were in a neutral environmental temperature (15–20°) and while acutely exposed to a moderately cold (1°, wind speed 2 m/s) environment.

2. Before and at various times after feeding measurements were made of hepatic portal blood flow and the concentration of volatile fatty acids (VFA) and glucose in arterial, hepatic portal and hepatic venous blood plasma. From these measurements the net rate of release of VFA from the viscera was calculated, and the uptake of propionate and output of glucose by the liver was estimated, assuming hepatic arterial blood flow to be 20% of portal flow. The concentration of insulin in arterial and portal venous plasma was also measured.

3. The change in environmental temperature did not affect the time taken by the animals to eat the meal completely.

4. After feeding, in the neutral environment, there were significant increases in portal blood flow and release of VFA into the portal bloodstream. The uptake of propionate by the liver increased, significantly, and output of glucose also increased, but not significantly. Plasma insulin concentration also increased after feeding.

5. During cold exposure portal blood flow was consistently higher, before and after feeding, than it was in the neutral environment. The release of VFA into the portal blood was also consistently greater during cold exposure, especially the release of propionate after feeding. Associated with this was an extra uptake of propionate and output of glucose by the liver. Plasma insulin concentration was slightly higher in the cold environment than the neutral environment before the animals were fed, but this difference was not apparent at any other time.

1. The syndrome induced by depriving cats of animal lipid is partially cured by feeding 18:3ω6. This is associated with an increase in levels of 20:3ω6, but not 20:4ω6, in plasma phospholipids.

2. It is concluded that the cat lacks Δ5 desaturase activity and has a dietary requirement for 18:3ω6 and possibly 20:4ω6.