The relationship between serum dopamine-beta-hydroxylase (DBH) activity and sympathetic nervous system activation in response to insulin-induced hypoglycemia was investigated in healthy young male and female volunteers. Serum DBH activity and glucose levels were measured in blood samples obtained 30 min before and 90 min after the iv administration of regular insulin or placebo. All subjects developed significant hypoglycemia in response to the insulin and manifested signs of sympathetic activation, including increased heart rate, diaphoresis, and lightheadedness. However, despite obvious clinical manifestations of increased sympathetic activity, none of the subjects exhibited a significant alteration in serum DBH activity either in comparison to baseline or placebo values. These data question the suitability of serum DBH measurements for assessing adrenomedullary function in man.