Abstract. The stimulation of meiotic maturation of starfish oocytes by the hormone 1-methyladenine is mimicked by injection of 8)' subunits of G-proteins from either retina or brain. Conversely, the hormone response is inhibited by injection of the GDP-bound forms of O/il or O/t subunits, or by injection of phosducin; all of these proteins should bind free/33'. t~-subunit forms with reduced affinity for 87 (O/il or O/t bound to hydrolysis-resistant GTP analogs, or t~irGMPPCP treated with trypsin to remove the amino terminus of the protein) are less effective inhibitors of 1-methyladenine action. These results indicate that the /33, subunit of a G-protein mediates 1-methyladenine stimulation of oocyte maturation.