A radioiodinated superagonist analog of gonadotropin-releasing hormone (GnRH:[D-Ser(TBu)6]des-GLy10-GnRH N-ethylamide) was used to quantitate the GnRH receptor content of single pituitary glands. This ligand binds with high affinity (Ka = 4.9 X 10(9) M-1) to a single class of sites in pituitary homogenates, without significant tracer degradation, during equilibration for 80 min at 4 C. The GnRH receptor content of adult male rat pituitaries [112 +/- 6.4 (SE) fmol/gland; n = 9] was similar to that of adult female rat pituitaries obtained at estrus or metestrus (104.8 +/- 6.4 fmol/gland, n = 13). Between 1800 h on metestrus and 1800 h on diestrus, the pituitary content of GnRH receptors increased to 200 fmol/gland or greater in each of three separate experiments. The pituitary content of GnRH receptors remained elevated until the time of the serum LH surge, then consistently fell sharply to metesterus levels. These data imply a physiological relevance for the GgRH-binding sites measured by radioassay and demonstrate their relationship to the events of the estrous cycle. The increased GnRH receptor content from the evening of diestrus until late in the afternoon of proestrus may contribute to the enhanced sensitivity of the pituitary to GnRH during proestrus. The close temporal relationship between rising blood estrogen levels and increasing pituitary GnRH receptors suggests that steroid-mediated receptor induction occurs before proestrus. Such an effect of estrogen could be exerted directly upon the pituitary gland or indirectly via the hypothalamus to increase the release of endogenous GnRH, which subsequently induces its own receptors in pituitary gonadotrophs.