A high performance liquid chromatography (HPLC) method was developed and validated for the determination of 7-chloro-3-[[(N,N-dimethylamino)propyl]amino]-2-quinaxolinecarbonitri le 1,4-di-N-oxide (Q-85), a new hypoxia-selective agent, in plasma. The assay involves extraction into diethylether-dichloromethane (2:1) from plasma and subsequent analysis by reversed-phase HPLC with ultraviolet detection at 292 nm. Response was linear in the range of 0.4-20 micrograms/ml of plasma and the detection limit was 0.05 micrograms/ml. The extraction recovery was about 90%. The within and between day coefficients of variation did not exceed 6.1%. This HPLC assay has been applied to determine Q-85 in plasma samples taken during pharmacokinetic studies in mice.