The effects of calcium on fasting plasma insulin and glucose levels were compared in 16 normal subjects and 11 patients with β-cell neoplasms of the pancreas. Calcium was administered iv either as a rapid calcium infusion (RCI; 2 mg/ kg in 1 min) or as a long calcium infusion (LCI; 12 mg/kg in 3 h). In normal subjects, the RCI produced a rise in mean plasma insulin from 11 ± 1 (±SEM) µU/ml basally to a peak of 18 ± 2µU/ml (P < 0.001). No consistent pattern of change in insulin levels occurred during the LCI, and plasma glucose levels did not change significantly with either test. In the patients with β-cell neoplasms, the RCI resulted in a rapid increase in mean plasma insulin from 36 ± 6 µU/ml to a peak level of 312 ŷ 67 (P < 0.002). With the LCI, a more gradual rise in insulin from 35 ± 11 to 92 ± 36 µU/ml occurred (P < 0.002). The mean increase in insulin in the patients with β-cell neoplasms was significantly greater for the RCI than for the LCI (P < 0.01). Pronounced increments in plasma insulin occurred in all 11 patients after the RCI, but in only 3 of 8 patients during the LCI. Plasma glucose levels declined significantly from 69 ± 7 to 56 ± 8 mg/dl during the RCI (P < 0.05) and from 69 ± 8 to 49 ± 7 mg/dl during the LCI (P < 0.005). Symptomatic hypoglycemia developed in 3 patients during the LCI but did not occur after the RCI. These data indicate that calcium is a more effective insulin secretagogue in patients with β-cell neoplasms when administered as an RCI than as an LCI, and suggest that the RCI may be a useful test for the diagnosis of insulin-secreting tumors. (J Cline Endocrinol Metab62: 210, 1986)