Vascular reactivity in-vivo and in-vitro was examined in rats with acute renal failure produced by bilateral nephrectomy or intramuscular glycerol injection. Bilaterally nephrectomized rats displayed enhanced pressor responses to noradrenaline and angiotensin. However, the contractile responses to noradrenaline, angiotensin and potassium chloride of aortic rings and portal vein segments from nephrectomized rats were not significantly different from the responses obtained in vessels from sham-operated controls. Rats with glycerol-induced ARF which were pretreated with indomethacin had significantly lower pressor responses to noradrenaline and angiotensin than similarly treated control animals. Aortic rings from glycerol-injected rats produced significantly smaller contractions to noradrenaline than preparations from controls. This difference was not abolished by incubation of vessels with indomethacin. The findings suggest that the absence of kidneys or the presence of damaged renal tissue and not uraemia itself have pronounced but opposite effects on vascular reactivity. The depression of vascular reactivity in glycerol-induced ARF does not appear to be a result of increased production of prostaglandins.