Acute inflammation induces changes in liver proteins with an increase in synthesis of positive acute-phase proteins such as alpha1-acid glycoprotein (alpha1-AGP) and a decrease in synthesis of negative acute-phase proteins such as albumin. This is associated with muscle wasting, mediated by increased proteolysis and impaired protein synthesis. As protein metabolism can be altered in other situations (malnutrition, growth) by the form of the dietary nitrogen, we studied the effects of the molecular form of nitrogen on liver and skeletal muscle adaptation, looking at gene expression for two acute-phase proteins (albumin and alpha1-AGP) and a number of muscle proteins (alpha1-actin, ubiquitin and C9 proteasome subunit). Two groups of 24 Wistar rats (250 g) were injected S/C with 0.125 ml turpentine/rat and were fed one of two liquid diets. These diets had caloric, nitrogen, carbohydrate and lipid content but differed in the molecular form of the nitrogen source (whole protein [WP] versus peptide hydrolysate [PH]). Liver and muscle adaptation were studied at 18, 42 or 66 h after turpentine injection. Weight, deoxyribonucleic acid and protein content of the liver were significantly higher with the WP diet than with the PH diet at 42 h and 66 h. There was more alpha1-AGP messenger ribonucleic acid (mRNA) at 18 h and less albumin mRNA at 42 h. Thus, the PH diet causes a more rapid increase in alpha1-AGP mRNA content and a smaller decrease in albumin mRNA content after turpentine injection than the WP diet. However, the changes in plasma acute-phase proteins (albumin and alpha1-AGP) were similar with the two diets. In skeletal muscle, there was no change in mRNA levels for the C9 proteasome subunit at any time point with both diets compared to the controls. However, there were greater ubiquitin mRNA levels at 18|h and less alpha-actin mRNA levels at 18 h, 42 h and 66 h following turpentine injection in the two dietary groups than in the controls. These results suggest that the molecular form of nitrogen ingested regulates hepatic gene transcription or mRNA stability of acute-phase proteins, during the early period of inflammation, but did not affect the expression of muscle proteins, which was altered by turpentine injection. Post-transcriptional control of acute-phase protein genes may contribute to the maintenance of similar plasma levels.