Membrane progestin receptor (mPR) α on the cell membrane of the oocyte is involved in the meiotic maturation of vertebrates, including teleosts, but little is known about the role of this membrane-bound follicular receptor. We investigated the ovarian expression of membrane progestin receptor (mPR) mRNA in medaka. In follicles that were destined to ovulate, transcripts of mPRα and mPRγ were expressed in the oocytes as well as the granulosa cells. Transcripts of mPRα and mPRγ were expressed at relatively constant levels in the whole ovary and in the preovulatory follicles throughout the 24-h spawning cycle. In vitro incubation of the preovulatory follicles with recombinant medaka luteinizing hormone caused no significant changes in the expression of mPRα and mPRγ mRNA, suggesting LH-independent follicular expression of these mPR genes. Using HEK293T cells expressing medaka mPRs, forskolin-elevated intracellular cAMP levels were found to be reduced on treatment of the cells with ligand 17α, 20ß-dihydroxy-4-pregnen-3-one (DHP), but only in the cells expressing mPRα. These results indicate that activation of mPRα and mPRγ with DHP may cause differential effects on the granulosa cells. Information obtained from the present study may help to elucidate the role of mPRα and mPRγ in the granulosa cells of the follicles.