The objectives of this study were 1) to determine how cellular growth of skeletal muscle is altered by the repartitioning agent cimaterol and 2) to determine if cimaterol alters endocrine status in association with its repartitioning effects. Thirty Dorset wether lambs were randomly assigned to a pre-treatment baseline group or received 0 or 10 ppm cimaterol in a complete, mixed, high-concentrate diet for 7 or 12 wk. Weights of biceps femoris (BF), semimembranosus (SM) and semitendinosus (ST) muscles were 32.8, 27.1 and 31.5% greater, respectively, in treated lambs at 7 wk, and were 22 to 24% greater at 12 wk. Longissimus (LD) cross-sectional area was 26 and 32% greater at these treatment intervals. Percent type I fibers declined significantly over the course of the experiment in ST, SM and LD, and cimaterol caused a small but significant reduction in percent type I fibers in the ST at 7 and 12 wk. Muscles from lambs fed cimaterol contained 50 and 75% more fibers that exhibited negative staining for phosphorylase activity. Mean cross-sectional area of type I and type II fibers in the combined portions of the ST were 30.4 and 29.3% greater, respectively, in cimaterol-fed lambs after 12 wk, while type I and type II fiber areas in the longissimus were only 13 and 15% greater, respectively. Cimaterol-induced hypertrophy of the ST resulted in both protein and RNA content being 30 to 35% greater (P less than .01) at 7 and 12 wk, while DNA concentration was 22% less (P less than .01) at 7 wk. DNA concentration returned to normal by 12 wk. These results indicate that cimaterol elicits a rapid increase in muscle RNA and protein accretion without concurrent incorporation of satellite cell nuclei. Plasma insulin and insulin-like growth factor-1 (IGF-1) concentrations were 55 and 34% lower, respectively, in cimaterol-fed lambs. Plasma somatotropin concentration and area under the curve were 2.3 times greater (P less than .01) in lambs fed cimaterol for 6 wk, while plasma cortisol, prolactin and glucose concentration were unaffected at 6 or 12 wk. The significant changes in endocrine status may be important in the mechanism(s) of cimaterol in altering muscle accretion.