1. Thigh muscle samples for analysis were obtained from 35 control subjects and 12 severely ill patients, of whom five had disease-induced undernutrition and seven had associated sepsis. An improvement in the Bergström technique for analysing muscle needle biopsy samples was used.
2. Muscle water, chloride and DNA were increased in both groups of ill patients, indicating an increase in the extracellular component and loss of muscle protein.
3. Falls in the concentrations of glutamine, glutamate, arginine, histidine, ornithine and lysine and rises in those of glycine, serine, asparagine, tyrosine, valine, leucine, isoleucine, phenylalanine, methionine and tryptophan were observed in the group of ill patients compared with those in the control subjects. The changes were present mainly in the patients with sepsis, significant differences between these and the undernourished ones without associated sepsis being observed in glutamine, lysine, tyrosine, valine, leucine, isoleucine, phenylalanine and methionine. The changes were either absent or slight in plasma.
4. The increased concentrations observed in six essential amino acids in the septic patients, but not in those with undernutrition, is consistent with the increased catabolic rate of sepsis.
5. However, the branched-chain amino acids, and lysine, threonine, arginine, and histidine, were present in lower concentrations than might be expected from catabolism, presumably due to utilization. It is suggested therefore that septic patients might benefit from increased proportions of these amino acids in their feeding regimen.