Omeprazole, a substituted benzimidazole and a new gastric acid inhibitor, has been determined in plasma and urine, together with three of its metabolites--the sulphide, the sulphone and the hydroxy compound. The methods comprise extraction from the biological materials with methylene chloride, followed either by direct injection of the extract onto a normal-phase liquid chromatography column or evaporation, dissolution and injection onto a reversed-phase system. The compounds were detected using ultraviolet spectrometry. The absolute recoveries obtained were mostly above 95%. The minimum determinable concentration for omeprazole was 20 nmol/l in plasma (relative standard deviation 10-15%) and 50 nmol/l in urine. The metabolites could also be determined at the same levels.