We have examined the effects of endothelin (ET) on the renal microcirculation by in vivo microscopy using the model of the split hydronephrotic rat kidney. ET, a potent vasoconstrictor peptide synthesized by vascular endothelial cells, showed marked and long-lasting effects on glomerular blood flow and vessel diameters in various segments of the renal vascular bed. Intravenously applied ET (100 ng/min/kg) increased systemic blood pressure from 123 +/- 7 to 156 +/- 4 mm Hg, decreased glomerular blood flow by 70%, and preferentially constricted larger preglomerular vessels, e.g. the arcuate artery. The competitive leukotriene antagonist FPL55712 significantly attenuated the vasoconstrictor response of the larger vessels. Local ET administration decreased glomerular blood flow in a dose-dependent manner (50% reduction at a concentration of 2.6 +/- 0.7 x 10(-9) M) and constricted smaller vessel segments, e.g. the afferent and efferent arterioles near the glomerulus. The constriction induced by ET was not significantly affected by the Ca2+ channel blocker nitrendipine (2.8 x 10(-6) to 1.1 x 10(-5) M). We conclude that intravenous ET effects are probably mediated by leukotrienes, inducing constriction of larger renal vessels. Locally administered ET acts directly on the renal vasculature, especially on smaller vessels.