A series of periodic increases in intracellular free calcium concentration ([Ca²⁺]_i) occurred upon fertilization in golden hamster eggs. The spatial distribution of the Ca²⁺ transients was investigated in single zona-free, aequorin-injected eggs, inseminated by single sperm. A supersensitive TV camera system for recording Ca²⁺-aequorin luminescence enabled us to observe the spatial distribution of the Ca²⁺ rise. In the first response, which usually occurred 10–30 sec after the sperm attachment, the increase in [Ca²⁺]_i began near the sperm attachment site, and the Ca²⁺ rise spread over the entire egg within 4–7 sec. The Ca²⁺ rise attained its peak in 5–8 sec, declined with almost even distribution, and ceased in 12–17 sec. The spreading Ca²⁺ rise was repeated in the second and sometimes the third response, starting from the same focus, but spreading more rapidly (∼2 sec). In succeeding responses [Ca²⁺]_i increased synchronously in the whole cytoplasm within 1 sec. When additional sperm attached to the egg after the occurrence of the first response by the first sperm, the spread of the Ca²⁺ rise could take place from near the site of additional sperm attachment but only in the second or third response.