A plasma assay for paracetamol by HPLC is described. Paracetamol is extracted with diethyl ether in the presence of an internal standard, beta- hydroxyethyltheophylline . The extracts are analysed using reversed phase chromatography (Radial Pak C18 column), the mobile phase consisting of a buffer mixture of 0.01 mol/l acetate (pH 4)-acetonitrile (920-80, v/v). The method is simple, rapid, precise and sensitive (lower level of sensitivity 0.13 mumol/l). It was used in a pharmacokinetic study in twelve volunteers.