A rapid, accurate, and sensitive high-performance liquid chromatographic (HPLC) method for simultaneous determination of venlafaxine (V) and O-desmethylvenlafaxine (ODV) in plasma and urine has been developed. V and ODV are extracted from plasma using a liquid-liquid extraction procedure, chromatographed on a Supelcosil LC-8DB column, and quantitated by UV detection at 229 nm. Linearity was established over the range 10–500 ng/ml for V and 7.2–720 ng/ml for ODV using 1.0 ml of human, rat, dog, and mouse plasma. For urine, for both analytes, an analytical range 0.1–10.0 μg/ml was established. Accuracy of >± 10% about the theoretical mean was achieved for all matrices, with intra- and interday coefficients of variation for precision of <10%. Endogenous components in plasma and/or urine or known metabolites of V do not interfere in the determination of the analytes. For both V and ODV a quantitation limit of 10 ng/ml for plasma was adequate for their estimation over a period of three half-lives, following administration of a pharmacologic dose in man, and the limit of 0.1 μ/ml, for urine, can monitor excretion of as little as 0.5% of the dose.