Thirty-two Border Leicester ♂ × Scottish Blackface ♀ wether lambs, aged about 5 months, were divided into two groups on the basis of live weight, such that group G1 contained the 16 lightest lambs and group G2 the 16 heaviest. Lambs in group G1 were subdivided equally at random either to be sham-implanted controls (Group C1) or to be implanted with 35 mg trenbolone acetate (TBA) + 5 mg oestradiol-17β (OE) (group T1) at 24 kg initial live weight. The lambs in group G2 were also subdivided into two groups (groups C2 and T2), and similarly treated approximately 1 month later at 37 kg initial live weight. The lambs were offered ad libitum a diet containing an estimated 12·5 MJ metabolizable energy (ME) per kg dry matter (DM) and 140 g/kg DM crude protein. Comparisons were made for the main effects of hormonal treatment and initial live weight. Both hormonal treatment and initial live weight gave increases for DM intake, gut fill, empty body weight, carcass weight and, in the half carcass side weight, weights of dissected lean tissue, bone and intermuscular fat and chemically determined DM, crude protein and lipid. Weights of mm. semitendinosus, longissimus dorsi, supraspinatus and gastrocnemius were also increased due to hormonal treatment and in group G2 lambs compared with those in group G1. When expressed as a proportion of carcass side weight, hormonal treatment effects were not significant for individual muscles and dissected carcass lean, bone and fat and chemically determined lipid and ash. Variable effects on other body components were recorded for both treatments in the absence of any significant interactions. The responses to hormonal treatment were essentially similar in groups G1 and G2. The question is raised as to the contribution of the greater food intake in implanted lambs to the maintenance of fatness in these animals.