An automated method based on column-switching reversed-phase in high-performance liquid chromatography the heart-cutting mode has been developed for the simultaneous determination of the two major human metabolites of tipredane, FPL 66365XX and FPL 66366XX, in human urine. The limit of quantification of the method was 25 ng/ml for both analytes from a urine injection volume of 100 microl. The intra- and inter-assay precision and accuracy were acceptable between 25 and 5000 ng/ml. No significant interferences were observed from either tipredane or a selection of its putative metabolites, or urine constituents in samples from male and female volunteers. Both analytes were found to be stable in human urine when stored at room temperature for two days, at 4 degrees C for six days, in a freezer at or below -20 degrees C for three weeks, and when the urine samples were subjected to three freeze-thaw cycles The method was unusual in that the initial separation was performed on a non-polar, octadecylsilane, column and the final separation on a more polar, trimethylsilane column. These columns were selected only after the investigation of a wide range of reversed-phase columns. The method's success was based on the greatly differing selectivities shown towards the two analytes by the organic modifiers, methanol and acetonitrile, present in the mobile phases used for the extraction and analytical stages