Plasma concentrations of FSH on Days 1, 2, 3, 8 and 9 of lactation (day of parturition = Day 0) were significantly higher than on the corresponding day of pseudopregnancy (day of vaginal plug = Day 1). Pituitary FSH content on Days 4-12 of lactation was also significantly higher than in pseudopregnant rats. However, pituitary LH content on Days 2-12 of lactation was significantly lower than in pseudopregnancy, whereas plasma concentrations of LH were almost similar. Two consecutive injections of LH-RH (100 ng/rat, i.v.), 60 min apart, resulted in a significant release of FSH in lactating rats (Days 3 and 15) in response to the second injection of LH-RH, but there was no increase in pseudopregnant rats on Day 3. The first and second injections of LH-RH produced significantly less release of LH on Days 3 and 15 of lactation than on Day 3 of pseudopregnancy. These results indicate that FSH secretion is not suppressed by suckling whereas the suckling stimulus is a potent inhibitor of LH.