The fusion of the limiting membrane of a secretory granule with the plasmalemma during exocytosis is equivalent to the fusion and release of contents that occurs when phospholipid vesicles fuse with planar bilayers. Experiments with bilayers demonstrate that phospholipid vesicles must swell if they are to fuse. Also, inhibition of exocytosis in solutions of high osmolarity occurs in several types of secretory cell. We report here experiments on the cortical granule exocytosis of sea-urchin eggs. Exocytosis is prevented when the osmolality of the medium surrounding the eggs is raised from 1 to 2 osmol kg-1. High osmolality also prevents calcium-dependent exocytosis in vitro. Prior treatment with calcium at high osmolality triggers fusion when normal osmolality is restored, even if calcium is removed before dilution. Addition of calcium causes the cortical granules to swell. The large increase in membrane capacitance which normally accompanies fusion is absent in eggs activated in solutions of high osmolarity. Our data are consistent with the idea that a secretory granule must swell to fuse with the plasma membrane and support the hypothesis of an osmotically driven fusion step during exocytosis.