Evidence has been provided for an increase in baseline serum corticotrophin (ACTH) levels in response to a rise in circulating ionized calcium (Cai) levels within the physiological range. In order to establish whether small Cai increments are also able to modify the basal secretion of arginine vasopressin (AVP), we infused calcium gluconate through an intravenous infusion pump in eight healthy male subjects (25-31 years old). Serum Cai, ACTH and AVP concentrations were measured every 10 min over an infusion period lasting 90 min. A significant progressive rise in serum Cai (baseline: 42 +/- 0.9 mg dL-1; 90 min: 47.2 +/- 0.9 mg dL-1, P < 0.001), ACTH (baseline: 30.7 +/- 1.3 pg mL-1; mean peak at 80 min: 37.4 +/- 2.4 pg mL-1, P < 0.01) and AVP levels (baseline: 2.1 +/- 0.6 pg mL-1; mean peak at 80 min: 3.2 +/- 0.5 pg mL-1, P < 0.01) was observed during calcium infusion. Furthermore, a significant positive correlation (r = 0.71; P < 0.001) was observed between ACTH and AVP responses to calcium infusion at 60, 70, 80 and 90 min. These data demonstrate that AVP secretion is stimulated by a slight rapid increase in serum Cai levels even though absolute serum Cai levels remain within the normal range. In addition, the positive correlation between Cai-induced ACTH and AVP increments suggests that AVP plays a releasing role on ACTH secretion during calcium infusion.