A rapid and specific high-performance liquid chromatographic (HPLC) assay for the quantitative determination of tinidazole in human plasma is described. After a simple extraction procedure, the compound is analyzed by HPLC using a reversed-phase column and a UV detector. Quantitation is accomplished using an external standard, and peak areas are determined with an integrating computer. The average recovery of tinidazole over a concentration range of 0.20-20.0 microgram/ml was 86.9 +/- 2.8% SD. The maximum sensitivity of the assay is approximately 0.10 microgram/ml; it is suitable for use in pharmacokinetic studies following administration of therapeutic levels of tinidazole to humans.