We report a HPLC-UV method for determination of p-nitrophenol (PNP) hydroxylation to 4-nitrocatechol (4NC) as a marker for CYP2E1 activity in rat hepatic microsomes. Proteins were precipitated by addition of 50 microL phosphoric acid (50%, v/v in water) to 500 microL microsomal suspensions. Following vortex mixing and centrifugation the supernatant (20 microL) was injected onto a Supelcosil C(18) column (150 mm x 4.6 mm, 5 microm), and mobile phase (22% acetonitrile, 0.1% trifluoroacetic acetic acid, 0.5% triethylamine) delivered at 1.0 mL/min produced resolved peaks for internal standard, 4NC, and PNP in < 11 min. Calibration curves were linear (r(2) = 0.999) from 0.1 to 40 microM with intra- and inter-day precision < 12% and accuracy >90%. The method's improved sensitivity (LOQ = 0.1 microM) and minimal sample processing allowed rapid monitoring of PNP hydroxylase activity in fetal, neonatal, juvenile, and adult rat livers.