A progressive fall in glomerular capillary plasma flow (QA) is observed in mercury chloride (HgCl2)-induced acute renal failure (ARF) although the site of the main lesion of this ARF is the proximal tubule. To elucidate this mechanism, we examined the expression of endothelin (ET)-1 and endothelial nitric oxide synthase (eNOS) protein at the glomerulus level in the kidneys of control rats and rats with HgCl2-induced ARF. Both ET-1 and eNOS protein were detected in the juxtaglomerular cells of afferent arterioles. The expression of ET-1 was significantly increased in ARF rats when compared to control rats. Inversely, the expression of eNOS protein was markedly reduced in ARF rats as opposed to control rats. These observations suggest the participation of the vasoconstrictor, ET-1 and the vasodilator, NO in a reduction in QA observed in HgCl2-induced ARF.