Isolated peritoneal mast cells from rats were observed, by phase contrast microscopy, to extrude secretory granules when exposed to 48/80 or the ionophores A-23187 and X-537A, which are known to facilitate transmembrane fluxes of calcium. These effects were abolished when the cells were treated with EDTA and suspended in a Ca-free environment. Ca-deprived cells exposed to any one of the three drugs promptly extruded granules when calcium, but not magnesium, was added to the incubation medium. Such Ca-evoked or Ca-dependent responses persisted when Na was omitted from the incubation medium and replaced with sucrose, choline, or K. The responses thus seem independent of possible shifts in the alkali metal ions. The results are considered support for the view that Ca influx mediates stimulus-secretion coupling and does so by initiating exocytosis.