We have examined the temporal relationship between intracellular Ca2+ concentration ([Ca2+]i) and secretion in single intact pancreatic beta-cells. Secretion was detected as the release of 5-hydroxytryptamine from pre-loaded beta-cells, using amperometry, and changes in [Ca2+]i were monitored by microfluorimetry. Stimulation of beta-cells by elevation of the extracellular K+ concentration ([K+]o), acetylcholine or glucose increased [Ca2+]i and, after a delay of 2-7 s, evoked amperometric currents. In the presence of glucose, we observed oscillations in [Ca2+]i which were associated with oscillations in the amplitude and frequency of amperometric currents: however, the temporal correlation was not exact, suggesting that there is a significant latency between the increase in average [Ca2+]i and exocytosis. Both the amplitude and frequency of the amperometric currents elicited by 50 mM KCl declined with successive stimulation, but were restored by agents which elevate intracellular adenosine 3':5':cyclic monophosphate (cAMP). This suggests that beta-cells may possess a readily releasable pool of granules which is replenished by cAMP. The variable amplitude of the amperometric currents is discussed in terms of a model in which several secretory granules fuse simultaneously with the plasma membrane.