The localization of cells which produce estrogen receptors in the rat central nervous system was performed using an in situ hybridization technique. For this, we used a synthetic oligonucleotide probe labeled with 35S complementary to the mRNA coding for a fragment (1-24) of estradiol receptor which is not common to the other steroid receptors so far characterized. Radioautography revealed that labeling was present in discrete brain areas. The results obtained with the in situ hybridization agree well with previous results obtained with other techniques. This approach should be very useful to study the effects of different factors on estrogen receptor gene expression in specific brain nuclei and individual neurons.