Abnormalities in plasma amino acid profiles have been reported in severe uraemia and dialysis patients and may be a consequence of altered protein metabolism in the presence of metabolic acidosis. We studied plasma amino acid profiles in 7 control subjects [GFR 92.7 ± (SEM) 14.5 ml/min/1.73 m2] and 7 elderly patients with renal failure (GFR 16.5 ± 1.3 ml/min/1.73 m2). Uraemic patients had significantly reduced plasma levels of valine, tyrosine, phenylalanine, tryptophan and elevated histidine compared to controls. There was no correlation between arterial pH or bicarbonate and plasma amino acid level