Two hypophysial partes distales were grafted under the kidney capsule of intact female rats. The plasma prolactin levels 15, 45 and 90 days after the operation were determined. At the same postoperative intervals the grafted glands of some of the operated rats were processed for conventional light and electron microscopy and for the demonstration of prolactin, FSH and LH according to the unlabelled immunoperoxidase procedure. The ultrastructural characteristics of the transplanted secretory cells and the amount and distribution of the immunoreactive material within their cytoplasm were used to evaluate approximately the secretory activity of these cells. Although levels of prolactin in the three experimental groups were significantly higher than those in control rats, a decrease in prolactin level was detected in 71% of the samples taken 45 days after operation. At day 15 the graft was completely surrounded by lymphoid cells whereas at day 45 these cells had invaded the whole graft. In the group sampled at day 90 the graft was free of lymphoid cells. When traced immunocytochemically the three types of cells followed different patterns of evolution after transplantation. Most prolactotrophs were hypertrophied in all groups but, in addition, they underwent a process leading to hyperplasia some time between days 45 and 90 after operation. Syncytial formations which probably correspond to multinucleated prolactotrophs were present only in the group sampled at day 90. The number of LH and FSH cells had decreased in the group at day 45 and by day 90 the former remained scarce but immunoreactive FSH cells were no longer found. At the ultrastructural level clear signs of involution of gonadotrophs and degradation by macrophages were seen in the graft 45 days after operation. The relation between the morphology and hormone content of the graft and hormone content of the plasma is discussed, together with several questions raised by the results. Pituitary transplantation can be used as an experimental model only if the time-dependent changes described here are taken into account.