Endothelin type 1 (ET-1) is an endothelial cell-derived 21-amino acid peptide with potent contractile effects on isolated vascular smooth muscle. The systemic hemodynamic effects of bolus intravenous injections of ET-1 and angiotensin II (ANG II, 300 pmol) were examined in anesthetized male Munich-Wistar rats by measurements of mean arterial (AP) and right atrial (RAP) blood pressures and cardiac index (CI, electromagnetic flowmetry) over a 60-min period. ET-1 induced a biphasic pressure response: transient hypotension occurred in the early phase with all doses, followed by a more prolonged dose-dependent elevation of blood pressure in the late phase. Because CI was unchanged during the early phase, the hypotension resulted from systemic vasodilation. On the other hand, the marked rise in AP produced by 300 pmol of ET-1 in the late phase was associated with a significant fall in CI, and thus total peripheral resistance index (TPRI) increased profoundly. A fall in right atrial pressure and significant hemoconcentration were associated with this pronounced vasoconstrictor effect, suggesting that a contraction of plasma volume contributed to the reduction of CI. Additionally, stroke and minute work indexes and peak flow velocity became significantly reduced in the late phase for the 300-pmol dose of ET-1. When compared with an equimolar dose of ET-1, 300 pmol of ANG II produced a prompt, more marked, but shorter-lived rise in AP with minimal changes in CI, TPRI, RAP, and hematocrit. These results raise the intriguing possibility that endothelin may play a role in both the control of normal vascular smooth muscle tone and in the pathogenesis of vasospastic disorders.