In the present experiment, clenbuterol was supplemented (.42 ppm) from Day 1 or from 2 or 4 wk of age until slaughter age (6 wk). The effects on growth performance and on plasma hormone and metabolite profiles were investigated at 2, 4, and 6 wk of age in male and female broilers. There were no consistent or cumulative effects of clenbuterol feeding on growth and feed efficiency. Clenbuterol feeding from Day 1, but not later, depressed subsequent feed intake. Relative abdominal fat pad weight was reduced profoundly and was even more pronounced after prolonged supplementation and for females. Fat content of thigh meat (including skin), but not of breast meat (without skin), was reduced by clenbuterol feeding. No consistent effects of clenbuterol supplementation on plasma thyroid hormones, growth hormone, insulin-like growth factor-I, and corticosterone levels were detected. Very low density lipoprotein (VLDL) concentrations were depressed in male broilers fed clenbuterol for 6 wk, but the plasma triglyceride level did not follow this pattern. There was no evidence for a consistent effect of clenbuterol on lipolysis in vivo, measured by plasma glycerol level. Between 4 and 6 wk of age, plasma VLDL and glycerol levels decreased in females but increased in males. This corresponds to the higher fat deposition in females. However, the most consistent effects were age-related changes in the plasma levels of most hormones and metabolites. From the present study, it seems that clenbuterol acts primarily on fat deposition, the extent being dependent on sex, location of fat, and duration of beta-adrenergic agonist supplementation.