Plaunotol, an acrylic diterpene alcohol, is a new antiulcer agent derived from the "plau-noi" plant and has been reported to stimulate the release of endogenous secretin in humans. We investigated the effect of plaunotol on postprandial gastrin release, comparing it to the effect of exogenous secretin in a physiological dose in eight healthy volunteers. Four sets of experiments were performed in each volunteer: (1) meal alone, (2) meal after intravenous ranitidine (50 mg), (3) meal after oral administration of plaunotol (320 mg) in addition to ranitidine, and (4) meal after ranitidine with simultaneous intravenous infusion of secretin (0.03 CU/kg/hr). The postprandial increase in plasma secretin concentration was significantly reduced by ranitidine, while postprandial gastrin release was markedly exaggerated. Plaunotol in combination with ranitidine significantly increased secretin release and inhibited gastrin release after a meal. Intravenous infusion of secretin resulted in significant suppression of postprandial gastrin release exaggerated by ranitidine. The present study indicates that plaunotol inhibits postprandial gastrin release by its unique secretin-releasing action.