High performance liquid chromatography has been found to be an accurate method for determination of ondansetron in plasma. Within a single assay, and between assays, the differences between theoretical and observed means obtained from analysis of spiked samples were low and good precision was obtained. The method has been used to assay several thousand samples from subjects included in pharmacokinetic studies. The pharmacokinetic profile of ondansetron has been established following single and multiple oral dosing and intravenous infusions.