A new method has been devised for the determination of nanomole levels of allantoin in human plasma. Allantoin was converted into xanthylallantoin, which was chromatographed on a reversed-phase silica gel using a mixture of acetonitrile-water (27:73) as mobile phase. The eluted compound was measured using an ultraviolet detector. The detection limit of the assay for plasma was about 100 ng/ml. This method was applied successfully to the determination of allantoin in human plasma after oral administration of 100 mg of aldioxa.