Castrated adult male and female and androgenized female rats (AS rats ) were injected i.v. with 3H-estradiol (E2). Nuclear uptake and retention of the 3H-steroid was examined in pituitary luteinizing hormone (LH) and prolactin (PRL) cells by the combined techniques of autoradiography and immunocytochemistry. About 80% of PRL cells were found to concentrate the radioactive steroid compound in all experimental groups, while 89%, 82% and 68% of LH cells were found to be labeled in AS rats, normal female and male rats, respectively. This suggests that there are subpopulations of LH or PRL cells that contain no or, if any, small numbers of E2 receptor. Statistical analysis revealed that PRL cells take up more radioactivity than LH cells in male rats, while there is no significant difference between female and AS rats. Variations in E2 uptake (coefficient of variation) was higher in LH cells than in PRL cells in male rats and in AS rats. In females, on t the contrary, coefficient of variation was larger in PRL cells. Thus the characteristics of nuclear uptake and retention of estradiol in LH and PRL cells appear to be modulated in part by neonatal androgen since the pattern found in AS rats is different than that found in AS rats is different than that found in normal male and female rats.