A sensitive and highly specific assay for riboxamide (TCAR) in human and canine plasma is described. The specificity of the procedure is derived from the method of sample preparation and a high-performance liquid chromatographic separation which utilizes the different selectivities of two columns. Partial separation of TCAR from plasma is achieved on a solvent-generated anion exchanger with silica gel as the solid support. The separation is completed by switching the eluent fraction containing TCAR from the first column to a second solvent-generated anion exchanger which has ODS-silica as its support. The relationship between the amount of drug injected and its peak height was linear over wide ranges of concentrations (0-10 micrograms/ml) and injection volumes (20-200 microliter). The limit of detection for TCAR in plasma was 40 ng/ml which can be detected by injecting 200 microliter of processed plasma. The recoveries from plasma were 100.2 +/- 0.9% and 101.3 +/- 2.3% when spiked at the 10 and 1 microgram/ml levels, respectively. The applicability of the method to pharmacokinetic studies was demonstrated by following the plasma levels of TCAR after intravenous administration in the dog.