Sixty-four Sprague Dawley (SD) rats were divided randomly into 4 groups of 16. The control group was injected with physiological saline (1 ml); group 1 was injected with pure porcine somatotropin (PST) daily, 1 ml per rat (1.2 mg/ml); group 2 was injected with PST liposome every 7 days (8.4 mg/ml); group 3 was injected with PST liposome every 14 days (16.8 mg/ml). Porcine somatotropin (PST) and PST liposomes were injected into the abdominal cavity of the rats for 28 days. Both PST and PST liposome stimulated the growth performance of rats. There were, however, significant differences in growth rate and IGF-I content between the treatment and the control groups (P<0.05); the growth effect of PST liposomes on rats was mediated by the sustained-release formulation used in this experiment. The effect of this formulation lasted more than seven days.