The simultaneous determination of pentoxifylline and its 5-hydroxy metabolite in human plasma was performed: 10 ng/ml pentoxifylline and 15 ng/ml metabolite were determined at a signal-to-noise ratio of 3. The recoveries from plasma at a 100 ng/ml level were 98.0 and 86.9% for pentoxifylline and the metabolite, respectively. The intra- and inter-assay coefficients of variation were less than 5% for both pentoxifylline and the metabolite. This method shows advantages over many other published extraction procedures prior to high-performance liquid chromatographic analysis in terms of its speed and ease of manipulation.