The peripheral mechanisms responsible for pressor response produced by microinjections of baclofen (GABAb agonist) into the nucleus tractus solitarii (NTS) of conscious rats were studied. Bilateral microinjections of baclofen (10 to 1000 pmol/100nL) produced a dose-related increase in mean arterial pressure (MAP) and heart rate (HR). The maximal response was observed after 15 minutes. Intravenous injection of prazosin decreased MAP to control levels. Subsequent treatment with Manning Compound (vasopressin receptor antagonist, i.v.) produced an additional decrease in MAP. In a different group of rats, vasopressin antagonist was injected first and the MAP was significantly decreased, however it remained elevated compared to pre-baclofen injection levels. Subsequent treatment with prazosin abolished the baclofen-induced pressor response. Reductions in baclofen-induced pressor response with prazosin treatment were followed by a reflex tachycardia in animals that received 100pmol/100nL dose of baclofen. The tachycardia was not observed with the dose of 1000pmol/100nL. The pressor response induced by microinjection of baclofen into the NTS of conscious rats may be produced by both increases in sympathetic tonus and vasopressin release.