A simple and sensitive HPLC-fluorescence assay was developed for the determination of a gastroprokinetic agent mosapride in small volumes of rat plasma. Samples (50 microL) were treated with 200 microL of the internal standard solution (cisapride, 0.1 microg/mL in acetonitrile). Chromatographic separation was achieved on a C(18) column by gradient elution with the mobile phase of acetonitrile-water containing 20 mM potassium dihydrogen phosphate, at a flow rate of 1 mL/min. Fluorescence was measured with excitation and emission set at 315 and 354 nm, respectively. The retention time was about 16 min for cisapride and 20 min for mosapride. No endogenous substances were found to interfere. The calibration curve was linear from 0.015 to 10 microg/mL. The lower limit of quantification was 0.015 microg/mL. The intra- and inter-day precision expressed as relative standard deviation did not exceed 7.7%, and the accuracy was within 4.7% deviation of the nominal concentration. The method was used successfully to investigate the disposition kinetics of mosapride in rats.