We have used the isolated planar cortex of sea urchin eggs to examine the role of osmotic forces in exocytosis by morphological and physiological methods. Electron micrographs of rotary-shadowed replicas show an en face view of exocytosis and demonstrate fusion of cortical vesicles to the underlying oolemma upon addition of calcium. Freeze-fracture replicas of rapidly frozen cortices reveal specialized attachment sites between cortical vesicles and the oolemma, and between the cortical vesicles themselves. We describe a novel light scattering assay for the kinetics of fusion which allows rapid changes of solutions and monitors exocytosis in real time. The rate and extent of fusion are found to be calcium dependent. The removal of calcium halts exocytosis. The validation of exocytosis in this system and development of tools for kinetic analysis allowed us to test predictions of the osmotic hypothesis of exocytosis: hyperosmotic media should inhibit exocytosis; calcium should cause vesicular swelling. Cortical vesicles were found to be permeant to sucrose, glucose, and urea. In media made hyperosmotic with 1.7 M sucrose, cortical vesicles were seen to shrink. Addition of calcium in hyperosmotic media led to a 10-fold decrease in the rate of exocytosis compared with the isotonic rate. The rate, while triggered by calcium, was no longer calcium-dependent. This slowing of exocytosis allowed us to photograph the swelling of cortical vesicles caused by calcium. Removal of calcium had no effect on subsequent exocytosis. Return of cortices to isotonic medium without calcium led to immediate exocytosis. These results are consistent with the idea that swelling of cortical vesicles is required for fusion of biological membranes.