Epiroprim, an analogue of trimethoprim, has been shown to potentiate the efficacy of dapsone in experimental parasitic infections. A simple and accurate HPLC method has been developed to estimate epiroprim in serum and brain. Blood and brains from mice were sampled 0, 30, 75, 120 and 240 min after 50 or 100 mg kg-1 oral gavage. The drug and added internal standard metoprine in serum and brain supernatant were isolated by solid-phase extraction (Superclean LC-SCX). The HPLC system consisted of a 150 x 4.6 mm Hypersil 5 microns ODS column. The mobile phases contained various proportions of acetonitrile, methanol and phosphate buffer (0.1 M). Peaks were detected by UV absorbance at 210 nm. Serum concentrations (mean +/- s.e.m.) of epiroprim were highest at 30 min for both 50 and 100 mg kg-1 doses, 173 +/- 20 and 207 +/- 25 ng mL-1, respectively, falling to 8 +/- 5 and 18 +/- 6 ng mL-1, respectively, at 240 min. Epiroprim concentrations in the brain correlated well with those in the serum, with levels of 223 +/- 69 and 265 +/- 21 ng g-1 falling to 10 +/- 10 and 31 +/- 11 ng g-1, respectively. Epiroprim is rapidly absorbed and distributed to the brain.