We compared the hemodynamic effects of continuous i.v. infusion of endothelin-1 and big endothelin-1 in anesthetized rats. Big endothelin-1 was fivefold less potent than endothelin-1 in decreasing cardiac output, heart rate, and stroke volume. However, big endothelin-1 produced a significantly larger increase in mean arterial pressure compared to endothelin-1 at doses that produced identical decreases in cardiac output. These findings support the hypothesis that the hypertensive effects of big endothelin-1 and endothelin-1 are produced by differential effects on systemic vascular resistances. © 1991.