Insulin-induced hypoglycemia causes an increase in plasma vasopressin concentration in man and rat. To assess the mechanism by which this occurs, the effect of hypoglycemia was studied in healthy adults. After insulin injection, a 7-fold rise in plasma immunoreactive arginine vasopressin to 8.2 +/- 3.6 pg/ml was observed in 10 normal subjects. This was associated with a rise in plasma sodium of 2 meq/liter, but no significant change in mean arterial pressure or hematocrit was observed. The significance of the plasma sodium rise was assessed by observing the vasopressin response to hypoglycemia in a patient shown previously to have a selective loss of the vasopressin response to osmotic stimulation. His plasma vasopressin rose from 1.6 to 12.5 pg/ml with no fall in blood pressure or volume. beta-Adrenergic blockade with propranolol before repeat insulin-induced hypoglycemia did not reduce the vasopressin response (peak plasma vasopressin, 8.1 +/- 1.7 pg/ml), despite suppression of PRA. Linear regression analysis showed that the rise in plasma vasopressin and the percentage decline in plasma glucose correlated significantly (r = 0.57, P less than 0.001). In conclusion, hypoglycemia releases vasopressin nonosmotically by a mechanism that appears to be independent of factors currently known to effect vasopressin secretion.