These studies examined whether the decrease in pituitary responsiveness to gonadotropin-releasing hormone (GnRH) observed during lactation in the rat results from a change in pituitary GnRH receptors. GnRH binding capacity was determined by saturation analysis using D-Ala6 as both ligand and tracer. During the estrous cycle, the number of GnRH binding sites increased from 199 +/- 38 fmol/mg protein on estrus to 527 +/- 31 fmol/mg protein on the morning of proestrus, whereas there was no change in receptor affinity (Ka, 6-10 X 10(9) M-1), During lactation, females nursing 8 pups on Days 5 or 10 postpartum had 50% fewer GnRH receptors (109-120 fmol/mg protein) than observed during estrus or diestrus 1 (199-242 fmol/mg protein) although receptor affinity was similar among all the groups. No deficits in pituitary GnRH receptors were observed in females nursing 2 pups on Day 10 postpartum. Removal of the 8-pup suckling stimulus for 24 or 48 h resulted in a dramatic increase in GnRH receptor capacity by 24 h from 120 +/- 16 to 355 +/- 39 fmol/mg protein. The rise in GnRH receptors after pup removal was accompanied by an increase in serum luteinizing hormone (LH) and estradiol concentrations. To assess the role of ovarian steroids in determining GnRH receptor capacity during lactation, females were ovariectomized (OVX) on Day 2 postpartum. Suckling of a large litter (8 pups) completely blocked the postcastration rise in serum LH and in pituitary GnRH receptors on Day 10 postpartum (OVX+ 8, 77 +/- 12 fmol/mg protein; OVX+ 0, 442 +/- 38 fmol/mg protein).(ABSTRACT TRUNCATED AT 250 WORDS)