Animal experiments have shown extensively that the endothelial generation of nitric oxide participates in the regulation of renal haemodynamics and function, probably as a factor modulating the effects of endogenous vasoconstrictors. Although the endothelial function of the systemic vasculature is assessed through the vasodilatory effect of acetylcholine, only L-arginine infusion is available for studies of the influence of nitric oxide on the renal circulation in humans. In addition to peripheral vessels, L-arginine is a potent renal vasodilator, having no effect on the glomerular filtration rate; and the renal relaxation induced by L-arginine is markedly blunted in patients with essential hypertension. A few studies showing the renal vasoconstrictor effect of an inhibitor of nitric oxide synthesis have been reported. (C) 1998 Rapid Science Ltd.