The in-vitro effects of oestradiol and testosterone, alone or together with an aromatase inhibitor (1,4,6-androstatriene-3,17-dione), on the appearance of progesterone receptor (PR) in the pituitary were studied in the chick embryo. Whole pituitary glands from 10-day-old embryos were cultured for 2 days in a hormonally defined medium. The oestrogenic activity of the hormone added to the medium was determined by the induction of PR in nuclei of pituitary cells. Cells containing PR were identified by immunohistochemistry with an antibody to PR. Pituitary cells contained PR in their nuclei after 48 h of incubation with oestradiol or testosterone (0·1 μmol/l). Dihydrotestosterone failed to induce PR in pituitary cells, as did testosterone if the aromatase inhibitor was added to the culture medium. It is concluded that testosterone is aromatized locally at the level of the pituitary and, consequently, acts through the oestradiol receptor to induce the appearance of PR in pituitary cells.