A continuous flow system has been developed to study the endocrine functions of isolated organs. The procedure has been used to investigate the effect of a synthetic gonadotrophin-releasing hormone (Gn-RH) upon the anterior pituitary gland of the adult male rat. A Radiometer blood gas/pH analyser was used to monitor the pH and partial pressures of oxygen and carbon dioxide in the perfusion medium. A series of six experiments with four pituitaries per flask established that the rate of gonadotrophin release under basal conditions was 646±301 (S.D.) ng LH/ml and 404±124 (S.D.) ng FSH/ml. The duration and intensity of the response to Gn-RH was assessed by measuring the areas under the curves according to the trapezoidal rule. A significant increase in the release of FSH and LH was obtained by the administration of 250 ng Gn-RH/ml medium, and dose-response curves were produced for up to 10 μg/ml.