Aspects of the lipid metabolism of Walker 256 carcinosarcoma-bearing cachectic rats (TB) were investigated during a 14 day interval of tumour growth. Food intake and body weight of the TB rats were reduced by 18% and 13%, respectively, on day 14, as compared with non-tumour-bearing animals. The tumour burden then, corresponded to 19% of total body weight. The total fat content was not different in the liver, heart, carcass, epididymal (EAT) and retroperitoneal (RPAT) adipose tissues of the two groups. The brown adipose tissue (BAT) and skeletal muscle (gastrocnemius-SM) of the TB rats had increased levels of fat (23% and 200%, respectively). Enteral absorption of 14C-triolein was decreased in the TB rats, but the liver, heart, and SM of these animals incorporated more radiolabelled lipid than the control animals, while the adipose tissues exhibited a decreased incorporation of radioactivity in relation to controls. More lipid was incorporated into the VLDL fraction secreted by the liver of TB rats, which exhibited a different distribution of the incorporated 14C-oleate in the various lipid subfractions. Ultrastructural studies showed that the hepatocytes of the TB rats had a greater incidence of lipid droplets in the cytoplasm.