After preincubation of rat anterior pituitary cells in culture for 48 h with 10(-9) M 17 beta-estradiol (E2), basal LH release is increased 1- to 2-fold, while the concentration of LHRH required to induce a half-maximal stimulation (ED50) of LH release is approximately 50% reduced. The presence of 10(-7) M progesterone (P) alone for 48 h has no effect on LH release, but it inhibits the sensitizing action of E2 on the LH response to LHRH. This inhibitory effect of P on the E2-induced increase of LH responsiveness to LHRH is similar to the effect of androgens. While not affecting the LHRH ED50 for FSH release, P stimulates both basal FSH release and the maximal FSH response to LHRH. This effect of P is potentiated by simultaneous incubation in the presence of E2. The stimulatory effect of E2 alone on LH release is exerted at an ED50 value of 1-2 x 10(-11) M. A similar ED50 value is found for the potentiating effect of E2 on the P-induced stimulation of FSH release. The stimulatory effect of E2 on LH responsiveness to LHRH appears to be due to changes in the sensitivity of the release mechanisms in gonadotrophs, since the total hormone content (release plus cell content) is not affected by E2 under the same experimental conditions. The stimulatory effect of P on FSH release is measured at an ED50 value of 1 x 10(-8) M and is maximal after approximately 8 h of incubation with the steroid. While not affecting total LH, P increases total FSH (release plus cell content), indicating that the stimulatory effect of P on FSH release could be secondary to changes in the hormone content of cells.