A simple, rapid method of determining the ibuprofen concentration in small volumes of human plasma (50 microl) by HPLC was developed. The sample was prepared for injection using a solid-phase extraction method, with naproxen as the internal standard. A 96-well extraction plate was used, easing sample preparation and allowing the simultaneous extraction of multiple plasma samples directly into the HPLC injection vials. Samples were stable at room temperature for at least 48 h prior to injection. The HPLC method used an ultraviolet detector with a 5-min run time and measured concentrations across the range typically seen with the clinical use of this drug. The calibration curve was linear across the concentration range of 0.78-100 microg/ml with a limit of quantitation (LOQ) of 1.56 microg/ml. The coefficient of variation for intra-day and inter-day precision was 6% or less with accuracies within 2% of the nominal values for low (4.5 microg/ml), medium (40 microg/ml) and high (85 microg/ml) ibuprofen concentrations. For ibuprofen concentrations at the LOQ, the intra-day and inter-day precision and accuracy were within 10 and 15%, respectively. Recovery was 87% or greater for ibuprofen. This method was used to analyze plasma samples for unknown ibuprofen concentrations in bioequivalence and limited food effect studies of different formulations of ibuprofen. Thus, this method has been fully validated and used in the analysis of unknown plasma samples for ibuprofen.