We have investigated lactate intolerance in nine patients with acute hepatorenal failue during 21 machine haemofiltration treatments using a lactate based replacement solution. In all cases hyperlactataemia occurred, the mean arterial lactate increased from 1±0.2 mmol/1 (mean ± SEM) prior to treatment to 3.2 ± 0.3 mmol/1 at 1 h (P<0.01), 4.2 ± 0.4 mmol/1 at 2h (P<0.01), 4.2±0.4 mmol/1 at 3 h (P<0.01) and 3.9 + 0.4 mmol/l (P<0.01) post-treatment. There were correlations between the maximum increase in blood lactate and both the change in arterial hydrogen ion concentration (r = 0.71, P=0.001) and the mean arterial blood pressure prior to starting treatment (r=-0.57, P= 0.007). During eight of the treatments (38%), the arterial hydrogen ion concentration increased. This group showed increased lactate intolerance in association with a lesser pretreatment mean arterial pressure. The administration of exogenous lactate to patients with hepatorenal failure who are at, or near to, the threshold of their own endogenous lacate metabolism can result in an increase in hydrogen ion concentration rather than the expected decrease, and therefore lactate-based dialysate solutions are best avoided. © 1990 European Dialysis and Transplant Association-European Renal Association.