Recombinant-DNA-derived bovine somatotropin has similar metabolic activities to pituitary-derived hormone in both laboratory animals (growth promotion), and in dairy cows (milk stimulation), but anabolic effects have not been investigated in ruminants. Although there is some controversy as to whether recombinant somatotropin has intrinsic lipolytic activity, the release of non-esterified fatty acids in vivo following a single injection (Hart et al. 1984) and the reduction in carcass fat after long-term treatment periods in sheep (Johnsson et al. 1986) indicate lipolytic and/or antilipogenic activity. This study was initially planned to examine these activities during long-term administration of recombinant bovine somatotropin in lambs. However, four cases of uriolitholiasis caused the proposed study of fat turnover to be abandoned and the following data are from those lambs which showed no symptoms of this problem.