Hypophysectomized female rats which received renal grafts of anterior pituitary (AP) or weight-matched intact controls were sampled under urethane anesthesia. Plasma growth hormone (GH) in sequential samples from each rat was measured by radioimmunoassay to determine the effect of exogenous thyrotropin-releasing hormone (TRH) on GH release from ectopic or intact AP. In a first experiment, following a baseline sample, a pre-treatment sample was taken from each rat 30 min after urethane injection, after which TRH (0.3 or 0.6 mug) or isotonic saline was injected iv, and samples were taken at 10 and 30 min post-treatment. Baseline GH levels in hypophysectomized-transplanted rats were in the range of 4.0 to 8.0 ng/ml, and were not modified significantly by urethane. TRH caused a significantly greater increase in growth hormone at 10 min than did saline. Plasma GH tended to be higher at 30 min post-treatment only in the 0.6 mug TRH-treated group. In further experiments the above described protocol was followed except that four doses of TRH were used (0.15, 0.3, 0.6, and 1.2 mug) and post-TRH blood samples were taken at 5 and 10 min. TRH caused a clear-cut increase in plasma GH both at 5 and 10 min, although no dose-effect relationship was present. In intact controls, baseline GH levels were in the range 40.0 to 80.0 ng/ml and were drastically reduced by urethane. In these animals, only the 1.2 mug TRH dose induced a GH rise at 5 and 10 min. In similar experiments, iv administration of vasopressin (100, 200, or 400 mU) induced a rise in plasma GH when given to the hypohysectomized-transplanted rats, but was ineffective in intact controls; the administration of prostaglandin E2 (5.0 and 50.0 mug) increased plasma GH in both experimental conditions. The results indicate that TRH in the hypophysectomized-transplanted rat acts directly on the AP tissue to increase GH release and that the ectopic pituitary is more susceptible than the in situ pituitary to some GH-releasing stimuli.