Exposure to estrogen or estrogenic metabolites of testosterone during the early postnatal period has permanent effects on rodent brain development. Differential sensitivity to estrogen, as reflected by transcription of the estrogen receptor gene, might determine the period of maximal sensitivity to the masculinizing effects of estrogen. We used an 35S-labeled riboprobe and in situ hybridization to chart the development of estrogen receptor (ER) mRNA expression in the rat preoptic area, a brain region for which sexual dimorphisms and the effects of estrogen on development are particularly well documented. Neonatal male and female rats were sacrificed by perfusion fixation on postnatal days 0, 2, 4, 7 or 10 (PND; day of birth is PND 0). Many ER mRNA-containing cells were detected in the periventricular preoptic area and medical preoptic nucleus and the distribution of ER-synthesizing cells was similar in both sexes. Analysis of film autoradiograms showed that the relative steady state level of ER mRNA was significantly higher in females than in males at all ages except PND 0 and 10. The temporal profile of ER mRNA expression was different in males and females. ER mRNA did not change with age in males, whereas in females, ER mRNA was significantly higher on PND 2 compared with PND 0 and 10. These results demonstrate that the pattern of ER mRNA expression is quantitatively and qualitatively different between the sexes during the neonatal period. The pattern of ER mRNA expression contrasts markedly with previous reports of estrogen binding based on biochemical and autoradiographic steroid binding assays.(ABSTRACT TRUNCATED AT 250 WORDS)