In an attempt to clarify the pathogenesis of the decreased branched-chain amino acid (BCAA) plasma concentrations in cirrhosis, the plasma clearances were measured in 7 patients with cirrhosis and in 7 age- and sex-matched control subjects. BCAA were given as prime-continuous infusions. The plasma clearances of valine, isoleucine, and leucine, calculated as infusion rate divided by steady state concentration, were low normal in cirrhotics despite hyperinsulinaemia, but different BCAA had different clearances (P less than 0.01). The endogenous basal appearance rates of BCAA, estimated by the basal concentrations multiplied by the plasma clearances, were lower in cirrhotics (P less than 0.025). The apparent theoretical volumes of distribution of BCAA, assessed by the ratio between the clearance and the concentration decay constant after infusion stop, were on average 67% of the total body weight, and were neither different among the three BCAA, nor between the two groups. The urea nitrogen synthesis rate did not increase significantly, suggesting that most of the infused BCAA nitrogen was taken up in peripheral tissues. The decreased concentration of BCAA in cirrhotics (394 +/- 81 mumol/l (mean +/- SD) in the present series vs 510 +/- 68 in controls; P less than 0.025) is not attributable to changes in plasma clearance. The most likely explanation is decreased afflux of BCAA into plasma.