The induction of oocyte maturation (OM) in fish by progestins is a valuable model for investigating interference of nongenomic steroid actions by endocrine-disrupting chemicals (EDCs). The potency of EDCs to induce or prevent the resumption of meiotic cell cycle by maturation-inducing steroid (MIS) was examined in vitro in oocyte culture. An endocrine-disrupting chemical, diethylstilbestrol (DES), a nonsteroidal estrogen, triggers oocyte maturation in goldfish and zebrafish. 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 MIS: 17α, 20b-dihydroxy-4-pregnen-3-one (17,20β-DHP). A synergistic action of DES on 17,20β-DHP-induced OM was observed. Both 17,20β-DHP- and DES-induced oocyte maturation was inhibited by an antibody against mPRα. The ability of EDCs to interact with the progestin receptor mediating OM was investigated in receptor binding assays using plasma membranes from goldfish ovaries and breast cancer cells transfected with goldfish membrane progestin receptor α (mPRα). Membranes prepared from both ovaries and mPRα-transfected cells showed high affinity, saturable, displaceable, single binding sites specific for the goldfish maturation-inducing steroid, 17,20β-DHP. DES and DES analogues (DP-DES and HEX), which induce OM in goldfish, bound to the receptor and caused concentration-dependent displacement of [3H]-17,20β-DHP. The relative binding affinities of various steroids and EDCs to these two receptor preparations were similar, supporting the idea that mPRα is a MIS receptor in vivo. The close correspondence between binding of DES and its analogues to the mPRα protein and their OM-inducing activities suggests a mechanism of endocrine disruption mediated by binding to mPRα resulting in its activation, thereby mimicking the nongenomic action of the progestin 17,20β-DHP.