Effect of various synthetic progestins and antiprogestational compounds on progesterone (P) and estradiol (E) production by isolated goat ovarian granulosa (G) and corpus luteum (CL) cell types was studied in vitro. Steroid production was studied either under basal conditions or after stimulation with follicle stimulating hormone (FSH) in the presence of aromatase substrate, androstenedione (A). None of the progestins had any significant effect on basal P and E production by either cell types during 48 hour of culture. The FSH and A - induced increase in P and E production was significantly inhibited following concommitant treatment with synthetic progestins at concentrations higher than 10(-7) M. The added progestins had no effect on G and CL cell viability. None of the antiprogestational compounds had any significant effect on basal steroid production in either of the cell types. Furthermore, the higher concentrations of three antiprogestins namely RMI 14156, STS 557 and isomer 201 of RMI 12936, were found to stimulate significantly the basal as well as FSH + A - induced production of estradiol in both the cell types. In contrast, the other two antiprogestins tested were found to stimulate the gonadotropin + A - induced production of P. These results indicate that exogeneous progestins directly inhibit the gonadotropin + androstenedione - induced steroid production by G and CL cells in vitro. Moreover, different antiprogestin had different effect on the modulation of steroid production.