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1. Sepsis was induced in rats by an intravenous injection of live bacteria. Infected and pair-fed animals were studied before the infection, in an acute septic phase (day 2 post-infection), in a chronic septic phase (day 6) and in a late septic phase (day 10). Protein synthesis rates were measured in vivo after administration of a flooding dose of L[1-13C]valine. 2. During the acute phase, muscle protein loss associated with infection resulted from both a decrease in protein synthesis and an increase in proteolysis. During the chronic phase and the late phase, the increase of proteolysis in infected rats as compared with pair-fed animals persisted, worsening muscle atrophy. Skin protein synthesis rates were not significantly modified by infection. However, skin protein content decreased 6 and 10 days after infection, suggesting an increased proteolysis in response to sepsis. 3. Protein synthesis in liver of infected rats was twice that of pair-fed animals. Liver protein synthesis remained elevated in infected rats compared with pair-fed animals until day 10. Hypoalbuminaemia and high plasma concentrations of fibrinogen were evident at all periods studied. alpha 2-Macroglobulin and alpha 1-acid glycoprotein reached peak concentrations during the acute phase (concentrations increased 50 times in infected rats). On day 10, the levels of these proteins were still about 12-fold higher. 4. Protein synthesis rates were significantly increased in the digestive tract and lung of infected rats compared with pair-fed groups on days 2 and 6, but were similar in the two groups on day 10 post-infection. The fractional protein synthesis rate was increased 3-fold over the entire experimental period in the spleen. 5. The results show that sepsis stimulates protein synthesis in various tissues over a long time, and that skin, like muscle, can provide amino acids to the rest of the body.