The areas S of the ventral surface of the brain stem and the immediately surrounding zone were superficially destroyed by the means of electro-coagulation, in 14 cats. This destruction produced a drop in blood pressure, which was transient in 9 and definitive in 4 animals; in one cat only the arterial pressure did not change after the destruction. In 6 animals which have been sham-operated, clonidine (15 mug/kg, i.v.) always induced a marked fall in blood pressure whereas in 10 animals which had maintained or recovered a normal blood pressure after the destruction of the area S, clonidine (15 mug/kg) injected intravenously no longer produced any decrease of the arterial pressure. These results suggest that the integrity of the areas S is necessary for the development of the hypotensive action of clonidine. This hypotensive drug may act, at least at the level of the ventral surface of the brain stem, through inhibition of a vasopressive structure.