Hemisected rat anterior pituitaries were incubated in KRB (pH 7.4) containing glucose (1 mg/ml) and BSA (1 mg/ml) and the release of growth hormone (GH) was measured by radioimmunoassay. Prostaglandin E1 and E2 (1 μg/ml each) increased GH concentration in the medium by 298 % and 266 % respectively over controls during a one-hour incubation period. On molar basis prostaglandins proved to be at least a thousand times more potent stimulants of GH secretion than theophylline or dibutyryl cyclic AMP. This response was drastically reduced by preincubating the pituitary explants in the presence of EGTA in a calciumfree medium. Addition of calcium (1 mm) restored the relative GH response to PGE1 though the absolute amount of GH released was considerably less than that observed without EGTA treatment. Hormone secretion was potentiated by high concentration of K+ (54 mm) which in itself significantly stimulated GH release. 2,4-Dinitrophenol almost completely abolished PGE1 stimulated GH release indicating that the latter is an energy dependent phenomenon. The possible mechanism by which prostaglandins stimulate GH secretion and the interaction of ions in the secretory mechanism are discussed.