1. Five Greyface wethers (42–45 kg) fed on various fixed amounts of dried grass pellets (either approximately 1·3 times maintenance or 2 times maintenance) by means of belt-type continuous feeders were housed in opencircuit respiration chambers for periods of 45 d. Between days 15 and 35 they received daily oral doses of 1·5 mg of the β-adrenergic agonist clenbuterol (adsorbed on to the feed). Continuous energy and nitrogen balance measurements each of 5 d duration were conducted throughout the chamber confinement.
2. On six occasions (twice during the 15 d pre-clenbuterol period, on days +4, +11 and +18 of clenbuterol administration and once during the post-treatment period) animals were infused with [l-14C]leucine to determine the rates of leucine oxidation and the amounts of leucine available for protein synthesis.
3. Clenbuterol administration caused a marked increase in N retention (2–3 g N/d; P < 0·001) throughout the 20 d treatment period. It also increased (P < 0·001) the energy expenditure of the animals (on average by 1·1 MJ/d over the first 5 d, compared with immediate pretreatment values, and 0·6 MJ/d over the 20 d period, compared with the mean of pre- and post-treatment control values). The effect of treatment was calculated to result, on average, in the daily retention of 19 (SE 1·5) g more protein and 30 (SE 5·5) g less fat.
4. During clenbuterol treatment leucine oxidation was reduced (P < 0·01). However, values for the amounts of leucine available for protein synthesis were equivocal, with an increase (P < 0·001) on day 11 of treatment, but no change on days 4 and 18.
5. Withdrawal of the clenbuterol resulted in rapid alterations of N and energy metabolism towards those expected of control animals of that weight.