The effect of long-term (10 weeks) treatment with growth hormone (GH) was investigated in twin lambs, one sibling being a control and the other treated with GH (0.1 mg/kg live weight per d). The lambs were fed on a concentrate-grass cube (9: 1 w/w) diet at a daily rate of 40 g fresh weightlkg Live weight. The average daily live-weight gain of the GH-treated lambs was 36% greater than that of the controls (307 v. 225 g/d, P < 0.01). The carcass composition of the GH-treated lambs changed: fat content was decreased (P < 001) and protein content was increased (P< 0.05) when expressed relative to carcass dry matter. The absolute weights and the weights when expressed relative to fleece-free empty body of some muscles were significantly increased in GH-treated lambs. The mean retention times of both particulate- and liquid-phase components of the digesta were unchanged by GH treatment, when calculated for the rumen or for the entire gastrointestinal tract. The feed conversion ratio was significantly greater (P < 0.01) in GH-treated lambs compared with controls. Nitrogen retained per g N intake was also significantly increased (P < 0.05) by GH treatment. Plasma urea concentrations were decreased (P < 005) and glucose concentrations were increased (P< 001) in GH-treated lambs, whereas non-esterified fatty acid concentrations were unchanged. Plasma insulin and total insulin-like growth factor-1 concentrations progressively increased in GH-treated lambs as treatment time continued. They were significantly correlated after week 4 of treatment. Two types of hepatic GH- binding site were detected, with high and low affinities for GH. The capacities of both binding sites were significantly increased (P < 0.05) in GH-treated lambs when expressed per unit microsomal protein but, when expressed per liver, only the capacity of the high-affinity site was increased.