OOCYTES of many species undergo parthenogenetic activation after exposure to the calcium ionophore, A23187, even in the absence of external Ca²⁺ (refs 1 and 2), whereas those of mouse, rat and hamster will activate simply on exposure to Ca²⁺ free medium^3,4. This suggests that Ca²⁺ is involved in parthenogenetic activation. Direct evidence of a transient increase in intracellular free Ca²⁺ at the time of fertilisation or activation of fish⁵ and sea urchin eggs⁶ has been obtained using the photoprotein aequorin. We report here that parthenogenetic activation and subsequent normal development can be induced in mouse oocytes by injecting them with Ca²⁺.