Phenelzine (PLZ), a nonselective irreversible inhibitor of monoamine oxidase (MAO), also inhibits GABA-transaminase (GABA-T), markedly increasing brain GABA levels. PLZ is also a substrate for MAO, and studies suggest that a metabolite formed by the action of this enzyme on PLZ may be responsible for the increase in GABA observed. We have recently found that PLZ also elevates brain ornithine (ORN), an amino acid precursor to both glutamate (and GABA) and the polyamines, and have conducted dose- and time-response studies on this effect. Rats were treated with vehicle or PLZ doses (7.5, 15 or 30 mg/kg i.p.), and brains were collected 3 h later. In the time-response study, animals were treated with vehicle or PLZ (15 mg/kg i.p.) and brains were collected 1–24 h later. To determine whether a metabolite formed by the action of MAO on PLZ may be responsible for the elevation in brain ORN observed, animals were pretreated with vehicle or the MAO inhibitor tranylcypromine (TCP) before vehicle or PLZ (15 mg/kg), and brains collected 3 h later. ORN levels (measured by an HPLC procedure) were dose- and time-dependently increased in PLZ-treated animals, with levels reaching approximately 650% of control at 6 and 12 h. Pretreatment with TCP completely abolished the PLZ-induced increase in brain ORN, suggesting, as with GABA, that a metabolite of PLZ formed by the action of MAO is responsible for the elevation of brain ORN observed. The possible contribution of increased ORN to therapeutic and/or neuroprotective properties of PLZ is discussed.