A high-performance liquid chromatographic method was developed for the determination of a new antiulcer agent, YJA-20379-1, in human plasma and urine, and rat blood and tissue homogenates. The sample preparation was simple: a 2.5-volume of acetonitrile was added to the biological sample to deproteinize it. A 50-microliter aliquot of the supernatant was injected onto a C18 reversed-phase column. The mobile phase employed was methanol--0.1 M Sørensen phosphate buffer of pH7.0--H2O (80:2:15, v/v/v), and was run at a flow-rate of 0.7 ml/min. The column effluent was monitored by ultraviolet detector at 254 nm. The retention time for YJA-20379-1 was approximately 6.3 min. The detection limits for YJA-20379-1 in human plasma and urine, and rat tissue homogenate (including blood) were 50, 100, and 100 ng/ml, respectively. The coefficients of variation of the assay (within-day and between-day) were generally low (below 9.44%) for the human plasma and urine, and rat blood and tissue homogenate. No interference from endogenous substances were found.