1. The pharmacokinetics of venlafaxine have been evaluated in mouse, rat, dog and rhesus monkey after i.v. and/or i.g. doses of venlafaxine from 2 to 120 mg/kg either as single or repeated doses. 2. In rat, dog and monkey, venlafaxine is a high clearance compound with a large volume of distribution after i.v. administration. 3. Absolute bioavailability was low in rat and rhesus monkey (12.6 and 6.5%, respectively) and moderate in dog (59.8%). Other species differences were seen, including an elimination half-life of venlafaxine that was longer in dog and rhesus monkey (2-4 h) than in rodent (around 1 h). 4. In mouse, rat and dog, exposure to venlafaxine increased more than proportionally with dose, suggesting saturation of elimination. Exposure of venlafaxine decreased with repeated dosing in mouse and rat, but was unchanged in dog. 5. Exposure of animals to the bioactive metabolite, O-desmethylvenlafaxine (ODV), was less than that of venlafaxine itself. ODV was not detected in dog and not measurable in rhesus monkey receiving venlafaxine.