1. The effect of dietary protein and energy restriction on heat production and growth costs has been examined in rats fed on a marginal (MP) or high (HP) protein diet, containing 9.2 % or 22 % respectively of the gross energy content as casein. Diets were given either ad fib. or at approximately 25, 50 or 75 % of the ad lib. intake.

2. Heat production (kJ/kg body-weight (W)0.75 per d) was increased by 23% in rats fed on the MP diet ad Lib., as compared with their HP controls (P < 0.01).

3. Factorial analysis of the data showed that the overall cost of energy deposition (kJ/kJ; Ee) was elevated on the MP diet (MP 1.7, HP 1.28; P < 0.001). Maintenance requirements (kJ/kg W0.75 per d) for zero energy balance were unchanged (MP 562, HP 573).

The partial energy cost of protein deposition (Ep) varied with dietary manipulation. If the partial energy cost of fat deposition (Ef) was assumed constant at 1.25 kJ/kJ, and maintenance requirements were assumed to vary with metabolic body size (W0.75), Ep was elevated on the MP diet. On both diets, Ep was reduced at low energy intakes.

5. The significance of these results is discussed in the context of current approaches to the analysis and interpretation of findings describing dietary induced changes in the rate of heat production.

1. The effect of corticosterone treatment on energy balance and heat production was investigated in growing rats. Animals were treated with daily subcutaneous injections of a vehicle containing 0, 50 or 100 mg corticosterone/kg for 5 d.

2. Measurements of digestible energy intake and urinary energy losses showed that corticosterone treatment resulted in a depression of metabolizable energy intake due to elevated urinary energy losses resulting from massive glucosuria.

3. Measurements of the metabolizable energy intake and the change in carcass energy indicated that at 50 mg/kg energy deposition and heat production were reduced, whilst at 100 mg/kg energy deposition was completely abolished with heat production increased. Postprandial oxygen consumption was unchanged at 50 mg/kg and increased at 100 mg/kg.

4. Factorial analysis of these results based on reported values for the energy cost of protein and fat deposition indicated that (a) the depression of total heat production at 50 mg/kg could be entirely accounted for by the concomitant suppression of growth, and (b) the elevation of total and postprandial heat production at 100 mg/kg reflected a specific influence of corticosterone on thermogenesis.

5. The significance of these findings is discussed in the light of reports that corticosterone in low doses suppresses heat production.

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.