Peptidergic neurotransmission is slow compared to that mediated by classical neurotransmitters. We have studied exocytotic membrane fusion and cargo release by simultaneous capacitance measurements and confocal imaging of single secretory vesicles in neuroendocrine cells. Depletion of the readily releasable pool (RRP) correlated with exocytosis of 10%-20% of the docked vesicles. Some remaining vesicles became releasable after recovery of RRP. Expansion of the fusion pore, seen as an increase in luminal pH, occurred after approximately 0.3 s, and peptide release was delayed by another 1-10 s. We conclude that (1) RRP refilling involves chemical modification of vesicles already in place, (2) the release of large neuropeptides via the fusion pore is negligible and only proceeds after complete fusion, and (3) sluggish peptidergic transmission reflects the time course of vesicle emptying.