1. In two separate trials male and female Wistar rats, 12 weeks of age, were either killed as a preliminary control group, ad lib.-fed or undernourished for 4 weeks until one-third of their 12-week body-weight was lost.
2. Food intakes, urinary and faecal collections and measurements of standard metabolic rate were made at one-weekly intervals on both the ad lib.-fed and undernourished animals of both sexes.
3. The bodies of the preliminary controls, the ad lib.-fed and the undernourished animals of both sexes were analysed for protein and fat, and the weights of four fat depots, two muscles and the major organs of all groups were determined.
4. Measurements of lipid synthesis rate (LSR) and lipoprotein lipase (EC 18.104.22.168) (LPL) activity in the four fat depots and measurements of whole-body protein synthesis rates were carried out on animals of both sexes in each group.
5. Although both sexes lost the same proportion of body-weight the females required more food on a body-weight basis than the males during the undernutrition period. The females absorbed significantly more energy on a body-weight basis during undernutrition and so were less efficient than the males at withstanding nutritional stress.
6. There were no significant differences between males and females, on a body-weight basis, in the excretion of nitrogenous waste products (urinary nitrogen, creatinine, hydroxyproline or NT-methylhystidine) suggesting that there were no differences between the sexes in protein sparing during undernutrition.
7. There were no differences between males and females in the proportions of body fat and protein used during the period of undernutrition or in the sites of the body from which the protein and fat were mobilized.
8. There were no differences between males and females in the way they responded to undernutrition by altering LSR, LPL activity or whole-body protein synthesis rates. Both undernourished males and undernourished females maintained synthesis of lipid, on a per g tissue basis and whole-body protein synthesis at the level found in well-nourished animals of the same sex.