A sensitive and selective high-performance liquid chromatographic assay for free and total ibuprofen and its major metabolites in human urine is described. Urine is acidified, drug and metabolites are extracted into hexane-propanol, back-extracted into sodium bicarbonate, neutralized and chromatographed. Ibufenac (4-isobutylphenylacetic acid) and 2-phenylpropionic acid were employed as internal standards. The extraction efficiencies were 94-100% for all compounds. The two metabolites and their internal standard were separated using an isocratic chromatographic system, followed by an abrupt step gradient to a second eluent for separation of ibuprofen and its internal standard with a total run time of 18 min. Detection was by a fixed-wavelength detector (214 nm). Sample-to-sample and day-to-day reproducibility studies yielded coefficients of variability of less than 9% for all compounds. The sensitivity was sufficient to determine 2.5 micrograms/ml free ibuprofen in 100 microliters urine.