AIM: To study the effect of rabeprazole (RAB) on nocturnal acid breakthrough (NAB) and nocturnal alkaline amplitude (NAKA) and to compare it with omeprazole (OME) and pantoprazole (PAN). METHODS: By an open comparative study, forty patients with active peptic ulcer were randomly assigned to receive one of the three PPIs (proton pump inhibitor) with a single oral dose. They were divided into RAB group (10 mg), OME group(20 mg) and PAN group (40 mg). Twenty healthy volunteers were enrolled to the control group (without taking any drug). Intragastric pH monitoring was then performed 1 hour before and 24 hours after the dose was given. RESULTS: No clinically undesirable signs and symptoms possibly attributed to the administration of RAB or OME and PAN were recognizable throughout the study period. All subjects completed the study according to the protocol. All data were processed by a computer using the Student test or test followed by an analysis of covariance. P<0.05 was considered to have statistical significance. The intragastric pH of NAB was significantly higher in RAB group (1.84