The effect of oestradiol treatment on the secretion of growth hormone (GH) was investigated in normal unrestrained male rats with chronic i.v. cannulae and in hypophysectomized male rats with autotransplanted pituitaries. The effect of gonadectomy of normal rats on the plasma GH secretory pattern was also evaluated. Baseline plasma GH levels were elevated following estradiol treatment of normal male rats (1.5 mg kg-1 per 15 days). Gonadectomy of male rats also resulted in increased baseline GH levels, although the effect was less apparent than after oestradiol administration. The pulse height was not influenced by gonadectomy or oestradiol administration. In male rats with the pituitary autotransplanted to the kidney capsule, oestradiol caused a dose-dependent increase in plasma GH levels, while there was no such effect of testosterone. These results suggest that the stimulatory influence of oestradiol on baseline GH levels is, at least partly, due to a direct effect on the pituitary. Plasma prolactin levels were elevated in rats with pituitary transplants receiving oestradiol. It is concluded that oestrogen administration to normal male rats increases baseline plasma GH levels, possibly by an effect exerted directly at the pituitary level.