A sensitive and selective high-performance liquid chromatographic assay has been developed for a new hypoglycaemic agent AG-EE 388 ZW (I) in human plasma. Plasma samples containing I were acidified with 0.2 M hydrochloric acid, directly injected into C2 reversed-phase pre-columns, and cleaned up on-line. After pre-column switching, the substance was separated isocratically in 15 min on a C18 reversed-phase column. Quantitation was performed after amperometric detection by external standard calibration curves. The linearity of the assay was demonstrated over the therapeutic concentration range 5-200 ng/ml. between-day coefficient of variation was 9.2% at 30 ng/ml. The limit of detection was 5 ng/ml in plasma. Determination of human plasma samples after intravenous and oral administration of 1 mg of 14C-labelled I demonstrated the applicability of the assay for pharmacokinetic studies in humans.