This experiment was conducted to determine the respective effects of bovine somatotropin (bST) and energy balance on the in vivo responses of plasma nonesterified fatty acids and glucose to isoproterenol (a nonselective [Beta]-agonist) or epinephrine injection in nonlactating nonpregnant cows. Two groups of adult Holstein cows were either underfed (n = 4) at 75%, or overfed (n = 5) at 150% of maintenance energy requirement, respectively. Cows received or did not receive a subcutaneous injection of Sometribove (500 mg) during two experimental periods (cross-over design). Adrenergic or placebo injections (4 nmol/kg body weight of epinephrine or isoproterenol or 4 mL of sterile saline) were administered intravenously on d 79 after bovine somatotropin injection, 1 h before or 3.5 h after feeding for under- or overfed cows, respectively. Glucose and nonesterified fatty acid responses to each challenge were calculated as area under the response curve and above the base line, from the time of challenge until 60 min postchallenge. Basal plasma nonesterified fatty acids and their response to adrenergic injections were enhanced by underfeeding. Responses of nonesterified fatty acids to isoproterenol injection were higher than they were to epinephrine injection. Basal plasma glucose was enhanced by bovine somatotropin treatment, which increased the glucose response at 5 min after adrenergic injections. Response of plasma glucose was higher after epinephrine than after isoproterenol injection. Treatment with bovine somatotropin did not change plasma nonesterified fatty acid responses to epinephrine or isoproterenol injection in under- or overfeed cows, at constant energy intake, whereas underfeeding modified these responses markedly. J. Nutr. 126: 945-954, 1996. INDEXING KEY WORDS: adult cows, undernutrition, somatotropin, epinephrine, nonesterified fatty acids