Membrane capacitance (C_m) was measured as an index of exocytosis in human growth hormone-secreting adenoma cells using the perforated whole cell, patch-clamp technique; the effects of membrane depolarization, growth hormone-releasing hormone, and 8-bromoadenosine 3',5'-cyclic monophosphate (8-BrcAMP) were examined. C_m was increased by membrane depolarization to potentials beyond the threshold necessary to open voltage-gated Ca²⁺ channels. These voltage-dependent changes in C_m varied as a function of both depolarization amplitude and duration and were blocked in the presence of the Ca²⁺ channel antagonist nitrendipine (10⁶ M). When membrane potential was clamped at the holding potential (78 mV), voltage-gated Ca²⁺ channels were closed, and neither application of growth hormone-releasing hormone nor 8-BrcAMP affected C_m. However, when these agents were applied to depolarized cells, where the voltage-gated Ca²⁺ channels were open, the increases in C_m were augmented. From these data, it was concluded that elevation of intracellular cAMP, per se, did not stimulate exocytosis. Rather, Ca²⁺ influx through voltage-gated channels was a prerequisite for cAMP-induced exocytosis.