Health and reproductive performance were observed in a lactational study involving 188 d of treatment with bovine somatotropin. Treatments commenced 84 ± 10 d postpartum and consisted of 27 mg/d of pituitary somatotropin and 0, 13.5, 27, and 40.5 mg/d of recombinantly derived somatotropin (six cows per treatment group). Cows were high yielding (>9600 kg/305 d), and somatotropin increased milk yield by 16 to 41% depending on source and dose. Somatotropin had no discernible effect on mammary health based on somatic cell count and incidence of clinical mastitis. Cows receiving somatotropin averaged 96% conception rate, 2.0 services per conception, and 116 d open, which were comparable to controls. No subclinical or clinical evidence of ketosis or milk fever was observed. Thirty-two blood chemistry and physical examination variables were examined at several intervals during the study and no adverse treatment effects of physiologic importance for any of the variables were observed. Somatotropin treatment had no discernible effect on gestation length or birth weight and growth rate (first 28 d) of calves. Milk yields for the first 60 d postpartum were compared for the period just prior to start of treatment and the lactation subsequent to treatment; no treatment effects were observed. Overall, our results for a well-managed herd demonstrate somatotropin treatment did not cause any serious health effects. However, examination of subtle health effects will require large numbers of animals treated under a range of environmental and management conditions.