Fertilization of mature mouse oocytes triggered highly repetitive Ca2+ oscillations lasting 2-3 h. However, immature oocytes generated only two or three oscillations, which ceased within 1 h. Development of repetitive Ca2+ transients to sperm occurred late in oocyte maturation and was dependent on cytoplasmic modifications that were independent of cell cycle progression from metaphase I to metaphase II. Immature oocytes released significantly less Ca2+ from stores than mature oocytes in response to ionomycin and thapsigargin. Ryanodine had no effect on intracellular Ca2+ in maturing oocytes hut stimulated an increase in Ca2+ in mature oocytes. The ability of ryanodine to increase Ca2+ levels was, however, strain-dependent. Preincubation of oocytes with thapsigargin or ryanodine significantly attenuated the normal fertilization Ca2+ response, causing a decrease in the number and the rate of rise of the transients. The inhibition of sperm- induced Ca2+ transients by ryanodine was independent of its ability to cause an immediate Ca2+ increase. Low concentrations of ryanodine had no effect on resting Ca2+ levels but inhibited Ca2+ oscillations at fertilization. Similarly Ca2+ oscillations were blocked in oocytes from a strain of mouse that showed no immediate Ca2+ increase with ryanodine. These results suggest that modifications in Ca2+ stores and ryanodine- sensitive Ca2+ release mechanisms during oocyte maturation play an important role in Ca2+ oscillations at fertilization.