Eighteen patients with fulminant liver failure were studied, 10 with normal renal function (group A) and eight with renal failure (group B, plasma creatinine greater than 200 mumol/l). Renal function was assessed by standard clearance techniques and patients in group B had a marked reduction compared with group A in both renal plasma flow and glomerular filtration rate. Raised plasma renin activity was observed in both groups, but levels in group B were significantly higher than in group A. Renal prostacyclin production was estimated by radioimmunoassay (RIA) of 6-keto-prostaglandin F1 alpha in urine, and the excretion rate was markedly increased in group A as compared with nine healthy controls, but was low in group B. The plasma concentrations of 6-keto-prostaglandin F1 alpha and thromboxane B2 were similar in groups A and B and were both significantly higher than in controls. Haemodynamic measurements showed a high cardiac output with low vascular resistance and mean arterial pressure within normal limits in both groups. The pulse pressure, however, was significantly higher in group B than in group A. In conclusion, patients in FHF with renal failure have marked renal vasoconstriction with increased plasma renin activity and reduced renal prostaglandin excretion indicative of an imbalance between vasoactive forces.