The human endotoxin model has been used to study the early phase of sepsis. The aim of the present study was to assess leg muscle protein kinetics after an endotoxin challenge given to healthy human volunteers. Six healthy male subjects were studied in the post-absorptive state before and during 4 h following an intravenous endotoxin bolus (4 ng/kg of body weight). Primed continuous infusion of [(2)H(5)]phenylalanine and [(2)H(3)]3-methylhistidine in combination with sampling from the radial artery, femoral vein and muscle tissue were used to assess leg muscle protein kinetics. Both two- and three-compartment models were used to calculate protein kinetics. In addition 26S proteasome activity and protein ubiquitination were assessed. An increase in the net release of phenylalanine from the leg following the endotoxin challenge was observed; however, this phenylalanine originates from the free intracellular pool and not from protein. Net protein balance was unchanged, whereas both protein synthesis and breakdown were decreased. Degradation rates of contractile proteins were not affected by endotoxin, as indicated by an unchanged rate of appearance of 3-methylhistidine from leg muscle. In addition, proteasome activity and protein ubiquitination were unaffected by endotoxaemia. In conclusion, intravenous endotoxin administration to healthy volunteers resulted in an increased release of free phenylalanine from skeletal muscle, whereas protein balance was unaffected. Both protein synthesis and breakdown were decreased to a similar extent.