1. Mixed-breed wethers (40–50 kg), 9 months old, were maintained on high-energy (HED) and low-energy (LED) diets for 2 weeks.
2. After a 15 h fast, a primed-dose constant infusion of L-[U-14C]leucine and α-[4,5-3H]ketoisocaproate (KIC) was given.
3. After 2 h, plasma samples were taken and plasma-specific radioactivities of I4C- and 3H-labelled leucine and KIC were measured and analysed by using an open two-pool model.
4. Less than 20% of the total leucine-C entering the circulation was converted to the KIC pool, and 42% of the KIC was converted back to the leucine pool; transamination of the leucine to KIC and reamination of KIC to leucine was much less than in other species.
5. Additional dietary energy resulted in a decrease in tissue protein synthesis, leucine oxidation and interconversion of leucine and KIC. Total leucine-C entry was also lower in sheep given HED, which was most likely due to a suppression of endogenous proteolysis.
6. Plasma glucagon concentration was significantly higher (P < 0.05) in sheep given LED compared with those given HED. The concentration of glucagon was closely correlated in all treatments with the leucine-C entry (proteolysis + absorbed leucine) and also with KIC-C exit (oxidation).