To determine how the sensitivity of the ectopic anterior pituitary gland to the GH-releasing effect of thyrotrophin releasing hormone (TRH) might be affected by the time lapse from transplantation, TRH (0-15 and 0-6 microng) was injected i.v. into hypophysectomized (hypox)-transplanted rats under urethane anaesthesia 1,3,8,15,30 and 60 days after transplantation, and plasma samples were taken 5 and 10 min later. Baseline GH values gradually decreased with time from about 16-0 ng/ml (1 day) to about 3-0 ng/ml (30 and 60 days). The TRH-induced GH release was absent 1 day after transplantation, present only with the higher TRH dose 3 and 8 days after transplantation, and clearly elicitable, also with the lower TRH dose (0-15 microng), from 15 up to 60 days. Determination of plasma prolactin concentrations showed a decline from about 85-0 ng/ml (1 day) to about 32-0 ng/ml (8 days); subsequently (15-60 days) prolactin values stabilized. Plasma prolactin levels increased 15 and 60 days after transplantation only when a dose of 0-6 microng TRH was given. In intact weight-matched rats, TRH induced a GH response only at the dose of 1-2 microng while a short-lived but clear-cut prolactin response could be obtained even with the 0-3 microng dose. The present results indicate that: (1) disconnexion between the central nervous system and the anterior pituitary gland greatly enhances GH responsiveness while blunting prolactin responsiveness to TRH; (2) the sensitivity of the anterior pituitary gland to the GH-releasing effect of TRH increases with time from transplantation; (3) TRH is a more effective prolactin- than GH-releaser on the pituitary gland in situ.