Gonadotropin secretion from the pituitary is regulated in large part by steroid action on the brain. An important question concerns whether luteinizing hormone-releasing hormone (LHRH) neurons themselves transduce steroid signals, or whether, alternatively, steroid-sensitive interneuronal populations regulate their activity. A previous study in the rat employing steroid autoradiography combined with LHRH immunocytochemistry revealed that only an exceedingly small percentage of LHRH-immunoreactive (ir) neurons was estrogen concentrating. This study has examined the relationship of estrogen receptive and LHRH-ir cells in the male and female guinea-pig brain with double label immunocytochemistry. Since estrogen receptor-ir, as demonstrated with antibody H222, is known to be confined predominantly to the cell nucleus, whereas LHRH-ir is localized mainly in the cytoplasm, single cells can be double-labeled. Diaminobenzidine tetrahydrochloride was used for localization of LHRH-ir while nickel-enhanced diaminobenzidine tetrahydrochloride was used for localization of estrogen receptor-ir. The results revealed that there were many brain nuclei that contained both LHRH and estrogen receptor-positive cells, including the preventricular periventricular nucleus, the anterior subcompact nucleus of the medial preoptic nucleus (MPNa), the remainder of the medial preoptic nucleus, the retrochiasmatic area, and the anterior, dorsomedial, ventrolateral and arcuate nuclei. However, of a total of 2,604 LHRH-ir cells that were examined, we observed only 5 double-labeled cells (<0.2%). The double-labeled cells were not restricted to a single nucleus; they were present in the MPNa, the retrochiasmatic area and the arcuate nucleus. Moreover, at the light microscopic level, LHRH cells quite frequently appeared to be apposed to estrogen receptor-positive cells (8.8% in the female), especially in the MPNa. These results lend further support to the notion that estrogen-mediated regulation of the LHRH system is achieved primarily through estrogen receptive interneurons. However, due to the existence of LHRH-LHRH synaptic interactions, the possibility also exists that a small population of estrogen-sensitive LHRH neurons could contribute to generalized activation of the LHRH system.