Intraven. injn. of concd, and purified influenza A virus into albino rats produced a pronounced drop of arterial blood pressure. The virus was harvested from infected chick embryo and purified by adsorption, elution from r.b.c, high-speed centrifugation and methanol precipitation. Injn. of control material obtained from normal chick embryo showed negative results. The hypotensive response was specifically neutralized by immune serum prepd. against the virus. The hypotensive factor was inseparable from the virus particles and it had greater resistance to heat inactivation than other properties of the virus such as hemagglutination, infectivity or mouse toxicity. Within limits, the degree of the hypotensive action was proportional to the hemagglutination titer of the virus prepn.