A sensitive and specific high-performance liquid chromatographic assay was developed for the simultaneous determination of p-aminohippuric acid (PAH), acetyl-p-aminohippuric acid (aPAH), and iothalamate in human plasma and urine. Plasma samples were prepared by protein precipitation with acetonitrile followed by evaporation, reconstitution in mobile phase, and injection onto a C reversed-phase column. Urine samples were diluted with 3 volumes of mobile phase prior to injection. 18 Column effluent was monitored by UV detection at 254 nm. The lower limits of quantification in plasma were 0.5 mg/ l for PAH and aPAH, and 1.0 mg/ l for iothalamate. The within-day and between-day coefficients of variation in plasma and urine were #7.8% for all analytes. This method is well suited for renal function studies using iothalamate and PAH, whether administered as a bolus dose or by continuous infusion, to measure glomerular filtration rate and effective renal plasma flow,respectively.