Antisecretory doses of prostaglandin analogues of the E1 and E2 series have been shown to promote peptic ulcer healing. In this double blind randomized cross-over study we investigated the effect of oral Rioprostil 300 micrograms b.i.d., 600 micrograms hs and placebo on 24 h intragastric H+ activity in 9 healthy male volunteers (age 20-37 years). 5-10 ml of gastric contents were aspirated hourly via a nasogastric tube. The pH of the samples was determined using a glass electrode . The results were expressed in terms of H+ activity (mmol/l). Night acidity (AUC/h, 2400-0800) is decreased by rioprostil in a dose related fashion (300 micrograms: 52%, 600 micrograms: 74%). The effect on day-time (0900-1800) H+ activity is much less. Rioprostil 300 micrograms given at breakfast reduce day-time acidity by 33%. Our results suggest that 600 micrograms Rioprostil in a single bedtime dose is effective in peptic ulcer treatment.