An endocrine-disrupting chemical, diethylstilbestrol (DES), a nonsteroidal estrogen, triggers oocyte maturation in fish. The morphology (the time course of the change in germinal vesicle breakdown) and an intracellular molecular event (the de novo synthesis of cyclin B) induced by DES were indistinguishable from those induced by a natural maturation-inducing hormone, 17alpha,20beta-dihydroxy-4-pregnen-3-one (17,20beta-DHP). A synergistic action of DES on 17,20beta-DHP-induced oocyte maturation was observed. Both 17,20beta-DHP- and DES-induced oocyte maturation was inhibited by an antibody against the maturation-inducing hormone receptor. The structural requirement for the action of DES is discussed based on results obtained with DES analogs.