The present study was undertaken to investigate the involvement of prostaglandins in the secretagogue action, observed after intraduodenal administration of rhein anthrone and rhein in rats. After intraduodenal administration of rhein anthrone (50 mg/kg), the active metabolite of sennosides, a very marked increase of secretion was observed compared to control. The amount of secretion was calculated by dividing the total weight of the small intestine, obtained 30 minutes after administration of the drug, by the total length. The effect seen with rhein (50 mg/kg) is far less pronounced than that with rhein anthrone and is not significant when compared with control. Pretreating the animals with indomethacin (15 mg/kg, p.o., 1 hour in advance) or with ibuprofen (15 mg/kg, p.o., 1 hour in advance) largely prevents the secretagogue effect of rhein anthrone, suggesting that prostaglandins play an important role in the observed pharmacological action. This idea is reinforced by the observation that pretreatment with hydrocortisone (50 mg/kg, p.o., 6 hours in advance) is also able to counteract the effect of rhein anthrone. After administration of rhein anthrone, an almost tenfold increase of the tissue content of prostaglandin E2 was observed. Here again, the results with rhein were far less pronounced. It is concluded that prostaglandins play an important role in the secretagogue action of rhein anthrone.