1. In Dahl S rats becoming hypertensive while on a diet of high NaCl content there appears to be a blood-borne humoral agent which produces vasoconstriction in a bioassay using the isolated hindquarters of rats.
2. This vasoconstrictor effect strongly suggests the presence of a humoral pressor agent or the lack of a vasodilator agent in the blood of hypertensive S rats.
3. The vasoconstrictor effect is not due to high renin concentrations.
4. This humoral vasoconstrictor action could partly account for the high vascular resistance in the hypertensive S rat.