We describe an isocratic reversed-phase liquid chromatographic method for the determination of venlafaxine (VLX) and its main active metabolite O-desmethylvenlafaxine (ODV) in serum, using haloperidol as internal standard and liquid/liquid extraction for sample preparation. VLX and ODV were separated on a C18 column with a mobile phase of acetonitrile/buffer (30/70, v:v) at 60 degrees C and a flow rate of 1.5 ml/min. The measurement of the native fluorescence signals of the eluted compounds were carried out at 227/300 nm (excitation/emission) without interference from endogenous components in serum. High linearities for VLX and ODV for concentrations between 20 and 500 microg/l were obtained (r = 0.9997). A large spectrum of routinely prescribed drugs did not interfere in the assay. The coefficients of variation for repeatability varied between 5.40% and 5.99% and for reproducibility between 9.43% and 21.63%. Absolute recoveries were more than 52% for both substances.