The influence of hypoglycemia on pituitary and cortical and medullary adrenal response was studied in Holstein X Friesian male calves by giving an intravenous injection of bovine insulin (0.2 U/kg b.w.). Seventeen (9 treated and 8 control) calves were born spontaneously at term (278 days) and 9 (5 treated and 4 control) were delivered by caesarean section on day 260 of gestation. Insulin injections were given 1, 5, 10 and 30 days after birth. Plasma insulin and ACTH concentrations were measured by radioimmunoassay, cortisol by competitive protein binding assay, catecholamines by radio-enzymology and glucose by colorimetry (glucose oxydase). In calves born spontaneously at term, the insulin injection induced a significant rise in plasma ACTH and cortisol concentrations on day 5 after birth; this rise occurred on days 10 and 30 after birth in calves delivered on day 260 of gestation. The rise in plasma insulin concentration following insulin injection was always more pronounced and sustained in calves delivered before term than in those born spontaneously at term. Following insulin injection, plasma epinephrine concentration increased significantly only on day 30 in calves born spontaneously at term. After treatment, no other change in plasma catecholamine concentrations was observed in any group of calves at any age. These results indicate that the pituitary-adrenal axis responded similarly in 5-day old calves born spontaneously at term and in 10 (or 30)-day old calves delivered by caesarean section 18 days before the expected time of parturition.