The effects on 24-hour intragastric pH levels of once-daily doses of lansoprazole 15 mg and lansoprazole 30 mg were compared with the effects of omeprazole 20 mg QD and ranitidine 150 mg QID in a phase I, randomized, double-masked, four-way crossover study conducted in 29 healthy male volunteers. Subjects received each treatment regimen for 5 consecutive days with at least a 2-week washout between treatment periods. Ambulatory 24-hour intragastric pH values were monitored in each subject at baseline (2 days before crossover period 1) and again before dosing on day 5 of each of the four crossover treatment periods. Gastric pH values increased during all four regimens, with significantly higher mean 24-hour pH values noted in subjects receiving lansoprazole 30 mg QD (4.53 +/- 0.16) compared with those receiving lansoprazole 15 mg QD (3.97 +/- 0.16), omeprazole 20 mg QD (4.02 +/- 0.16), or ranitidine 150 mg QID (3.59 +/- 0.16). Lansoprazole 30 mg produced significantly greater mean percentages of time that the gastric pH was above 3.0 and 4.0 (75% and 63%, respectively) compared with the other treatment regimens. The mean percentages of time during which gastric pH was above 3.0 and 4.0, respectively, for the other treatments were lansoprazole 15 mg, 64% and 48%; omeprazole 20 mg, 63% and 51%; and ranitidine 150 mg, 52% and 38%. All treatment regimens were well tolerated, with no clinically significant differences between the regimens. Multiple-dose lansoprazole 30 mg QD produced a significantly increased intragastric pH level and significantly longer durations of increased intragastric pH level compared with lansoprazole 15 mg QD, omeprazole 20 mg QD, and ranitidine 150 mg QID.