1. Forty-eight intact male lambs (30 kg) were fed to gain 16 (H), 5 (M) or –6 (L) kg during a 42 d interval (period 1). Lambs from each of the H and M groups were fed to gain either 16 (HH, MH), 5 (HM, MM) or –6 (HL, ML) kg and lambs from the L group were fed to gain 27 (LS), 16 (LH) or 5 (LM) kg during the ensuing 42 d (period 2).
2. Fasting heat production (FHP) of four lambs from each treatment was determined at the end of period 2.
3. Weights and compositions of the carcass, offal and digesta-free body as well as weights of major internal organs were determined for four lambs of each treatment at the end of periods 1 and 2.
4. Within groups of lambs of similar weight at the end of period 2, body composition was, in general, similar, but FHP was greater in lambs that had been on higher planes of nutrition during period 2.
5. Within groups of lambs of similar weight, lambs that were fed at higher planes of nutrition during period 2 had greater weights or proportions of liver, small intestine, large intestine and stomach.
6. Neither weight of the liver, kidney, stomach, small intestine, large intestine nor daily fasting heat production were constant functions of body-weight. Relations of these traits to body-weight changed with rate of gain.
7. Regression analysis indicated that the feeding of lambs at higher planes of nutrition during period 1 resulted in higher maintenance requirements of those lambs during period 2.