The effects of sex (wethers v. ewes), diet (chopped lucerne hay v. lucerne-barley pellets) and cimaterol on energy utilization by Suffolk cross lambs were determined by comparative slaughter. Quadratic regression of energy retention (RE) on metabolizable energy intake (MEI) enabled estimation of maintenance energy requirements (Em), efficiencies of gain (ktotal) and maximum rates of gain (REMAX). Regressions using RE in fat and protein v. MEI yielded analogous parameters for fat and protein deposition (Em fat, kfat, REMAX fat and Emprotcin, kprolein, REMAXprotcin respectively). Em was lower in wethers than ewes (455 v. 510 kJ/kg M0·75 per day respectively), but was unaffected by diet or cimaterol. Sex and cimaterol did not affect ktotai. which was higher in lambs given pellets compared with lambs given hay (0·417 v. 0·224 respectively). Similarly, REMAX was higher in lambs given pellets than in lambs given hay (326 v. 114 kJ/kg 0·75 per day respectively). None of the groups differed significantly in the parameters of fat deposition, which averaged 480 kJ/kg 0·75 per day for Em fal, 0·224 for ktat, and 250 kJ/kg M0·75 per day for REMAX, fat- Em.protein was lower in wethers than in ewes (466 v. 569 kJ/kg 0·075 per day, respectively), and was further reduced by cimaterol (418 and 507 kJ/kg 0·75 per day for wethers and ewes respectively). Estimates of kprotcin were higher in wethers than in ewes (0·091 v. 0·064 respectively), and were increased by cimaterol (0·115 and 0·089 for wethers and ewes respectively). Similarly REMAX protein was higher in wethers than in ewes (47 v. 37 kJ/kg 0·75 per day respectively), and was increased by cimaterol (58 and 48 kJ/kg 0·75 per day for wethers and ewes respectively). The repartitioning action of cimaterol was additive with effects of diet, intake and sex.