The effects of Revalor (trenbolone acetate plus oestradiol) implantation or the inclusion of clenbuterol (a beta-2-adrenergic agonist) in the diet of wether lambs was studied. Using continuous intravenous infusion of [3H]tyrosine the fractional synthetic rate of mixed protein from three separate muscles was measured. Clenbuterol slightly increased growth rate but had a significant (P less than 0.02) effect on food conversion efficiency. The weight and protein content of the longissimus dorsi and vastus lateralis muscles were increased but no such changes were observed for the vastus intermedius. For the longissimus dorsi at least the increase was probably achieved by a reduction in fractional degradation rate of the muscle protein. Revalor significantly increased the growth rate and food conversion efficiency of the animals. This increase was not specific for muscle. Estimated degradation rates of muscle protein were lower in the treated animals. The possible mode of action of these materials was discussed. The results obtained again highlight the importance of protein degradation in controlling growth.