E3810 is a new H+,K(+)-ATPase inhibitor with a substituted benzimidazole, which is under clinical investigation for peptic ulcer treatment in Japan and the USA. Three separate studies were conducted to evaluate the safety and to establish the pharmacokinetic profile of E3810 after oral administration to healthy male subjects. E3810 was administered as: single oral doses (1, 3, 10, 20, 40 and 80 mg) in fasting conditions, a single oral dose (20 mg) after a meal and repeated oral doses (20 and 40 mg) once daily for 7 days. The concentrations of E3810 and its metabolites in plasma and urine were determined by HPLC methods with UV detection. E3810 was generally well tolerated by all subjects. In the single-dose study, Cmax and AUC increased with increasing doses in the dose range examined. The mean plasma half-life was about 1.0 hour and was dose-independent. The apparent oral clearance of E3810 ranged from 4.37 to 8.40 ml/min/kg. No significant deviation from linear pharmacokinetics was observed. Approximately, 30% of a dose was excreted into the urine as thioether carboxylic acid-E3810 and its glucuronide. The mean serum protein binding was 96.3%. No effect of food intake on the Cmax and AUC was observed while tmax after a meal was 1.7 hours longer than that in the fasting conditions. No appreciable change in drug pharmacokinetics was observed during repeated oral dosing of E3810.