The effect of synthetic porcine endothelin on glomerular filtration rate, renal plasma flow and electrolyte excretion was studied in rats. Endothelin, 1 nmol/kg body weight given as a bolus, induced a transient decrease in glomerular filtration rate (72%) and in renal plasma flow (76%) as well as in sodium excretion, accompanied by a sustained increase in renal vascular resistance. This dose had no sustained effect on mean arterial pressure. It is concluded that endothelin induces a marked decrease in glomerular filtration rate and renal perfusion. This peptide could play a role in the alterations in renal function observed after renal injury.