A high-performance liquid chromatographic method for the determination of methoxsalen in plasma was developed. The method includes extraction from plasma of the drug and the internal standard (5-methoxypsoralen) into methylene chloride. Chromatography was performed on a reversed-phase C8 column connected with a UV detector set at 254 nm. The mobile phase was methanol-acetonitrile-water (2:30:68). For a plasma sample of 0.25 ml, the maximal sensitivity was approximately 10 ng/ml. Accuracy was within 7.5% for therapeutic plasma levels, and the coefficient of variation varied between 4.3 and 0.9% for 28 and 300 ng/ml of plasma, respectively.