Is calcium the second messenger of 1-methyladenine in meiosis reinitiation of starfish oocytes? Microinjection of EGTA into prophase-blocked oocytes does not inhibit hormone-induced meiosis reinitiation, although it prevents oocyte activation by fertilization, by ionophore A23187, or by subsequent microinjection of otherwise efficient Ca2+ buffers. In contrast microinjection of Ca2+ buffers inhibits 1-methyladenine-induced meiosis reinitiation. Oocytes can be released from Ca2+ inhibition by raising hormone concentration or by the subsequent transfer of cytoplasm taken from maturing oocytes. Ca2+-microinjected oocytes remain inhibited up to 1 h after microinjection, although free Ca2+ concentration comes back to its resting value less than 30 sec after microinjection. Cyanide, which decreases ATP content and depresses Ca2+-pumping activity, reversibly inhibits 1-methyladenine-induced meiosis reinitiation. These results do not support the hypothesis that Ca2+ is the second messenger of the hormone in meiosis reinitiation of starfish oocytes, although they support the view that elimination of Ca2+ from some component of the oocyte cortex (perhaps the plasma membrane) might be a compulsory event for transduction of the hormonal message.