One hundred and forty human fetal pituitary glands were removed from fetuses at 7-40 weeks of gestation and studied by light microscopy and immunocytochemistry to localize adenohypophysial hormones. For immunocytology, the avidin-biotin-peroxidase complex technique was more sensitive and identified hormones in younger fetuses than did the immunoperoxidase method. Adrenocorticotrophin, beta-endorphin, and growth hormone were the first hormones detected; they were identified by intense cytoplasmic immunopositivity at 8 weeks of gestation. Between 10 and 20 weeks, many growth hormone containing cells were large and showed scattered, faint positivity; after 20 weeks, smaller cells with intense positivity predominated. alpha-Subunit of the glycoprotein hormones was identified at 9 weeks of development; beta-subunits of thyroid-stimulating hormone, follicle-stimulating hormone, and luteinizing hormone appeared by 12 weeks. Gonadotrophs differed in numbers related to fetal age and sex. From 15 to 25 weeks, glands of female fetuses contained more gonadotrophs than did those of males; after 25 weeks, there was no significant difference in total gonadotroph numbers. Throughout gestation, adenohypophyses of male fetuses had more luteinizing hormone containing cells than follicle-stimulating hormone containing cells; pituitaries of females had approximately the same numbers of follicle-stimulating hormone containing and luteinizing hormone containing cells. Prolactin was identified in few small cells at 12 weeks; at term, prolactin-containing cells were numerous, comparable to those seen in the hyperplasia of maternal glands in late gestation and during lactation. This comprehensive study indicates morphologic correlations with pituitary hormone extraction data and with the appearance of the various hormones in the fetal circulation.