We have studied the effect on sea urchin eggs of ryanodine, a plant alkaloid that causes muscle contraction by opening calcium channels in the sarcoplasmic reticulum terminal cisternae. Ryanodine, although it is less effective that IP3 , produces full or partial activation in 62% of injected sea urchin eggs. In addition ryanodine inhibits in a dose dependant manner 45 Ca pumping in the isolated egg cortex or in eggs permeabilized with digitonin. Efflux experiments show that in fact ryanodine as IP3 stimulates the release of calcium sequestered intracellularly. We further show that these effects of ryanodine are inhibited by Mg++ , ruthenium red and heparin. Our results suggest that ryanodine-sensitive intracellular calcium channels exist in the sea urchin egg.