A hypotensive factor was isolated from the venom of Vipera aspis aspis by Sephadex G-75, S-Sepharose column chromatography, and reverse phase HPLC using a Develosil 300 ODS-7 column. The purified factor was a basic protein with a mol. wt of 25,000 and an isoelectric point of 7.95. Intravenous injection of hypotensive factor induced an immediate fall in blood pressure of rats, whose duration depended on the dose employed. The hypotensive response was not affected by dithiothreitol, beta-mercaptoethanol, EDTA, p-tosyl-L-phenylalanine chloromethylketone, p-chloromercuribenzoic acid, or diisopropyl fluorophosphate, and was resistant to heat-treatment at 100 degrees C for 30 min, however, it disappeared after incubation with antivenom prepared against the hypotensive factor. The factor is devoid of proteinase, esterase, phospholipase A2 and kallikrein-like activities, and lethal, hemorrhagic and capillary permeability increasing activities are also absent. Compared to the hypotension induced by the hypotensive factor in normotensive rats, a more potent response was observed when it was administered to 11-week-old spontaneously hypertensive rats.