High performance liquid chromatography in connection with monochromatic UV-detection has proved to be a powerful tool for separation and quantitative determination of drugs and their metabolites in body fluids. Serum samples from volunteers medicated with the psychotropic drugs fosazepam and nomifensine are analysed by this method. Separation times are less than 5 min; the detection limits are within the range of 30-50 ng/ml serum. The reliability of the method is discussed: The results are compared to those from the same serum samples obtained by measurement of total radioactivity and by quantitative mass spectrometry in order to confirm the accuracy of the method. Examples of the pharmacokinetic profile of the two drugs and their metabolites in serum are presented, based on analysis by the methods described.