Abstract Rats bearing the Yoshida AH-130 ascites hepatoma, a cachectic rat tumour, showed signs of important muscle wasting with reduced muscle weights. This phenomenon was associated with a decreased rate of in vivo alanine oxidation as measured by the production of 14CO 2 from [U- 14C]alanine intragastrically administered. It was later found that the decreased amino acid oxidation was associated with a reduced uptake in skeletal muscle as measured in incubated soleus muscles, thus suggesting that the decreased in vivo oxidation is basically due to a reduced oxidation of the amino acid in skeletal muscle. The decrease in alanine oxidation in the tumour-bearing animals was also associated with higher circulating alanine concentrations in their blood. In addition, tumour-bearing rats presented a lower (26%) protein synthetic rate in skeletal muscle, as measured by the incorporation of [ 14C]phenylalanine into muscle protein. The addition of insulin to the incubation medium abolished the lower rate of protein synthesis, thus suggesting a greater response to this hormone by the muscle of tumour-bearing rats. In conjunction with a reduced protein synthesis, tumour-bearing rats showed a clearly enhanced rate of protein degradation in isolated skeletal muscles. The results presented confirm previous observations suggesting that the skeletal muscle of tumour bearing animals is in a profound negative nitrogen balance which partially accounts for the wasting observed in the tissue. In addition, the present study allows us to conclude that, in spite of the increased alanine utilization for both gluconeogenesis and tumour growth, the oxidation of alanine by the whole animal is decreased in the tumour-bearing rats. This seems to be associated with a decreased ability of skeletal muscle to handle this amino acid.