Rats were fed diets containing 20, 50 and 80% protein for 14 months. The urea excreted by the rats fed diets containing 50 and 80% protein when compared to rats fed diets containing 20% protein increased ca. 2- and 3-fold, respectively, in ca. 2 days; this increase was maintained essentially unchanged through the experimental period. The serum levels of urea increased 2.5- and 4-fold, respectively, in the first days and were also maintained during the experiment. Glutamate dehydrogenase activity of liver remained unchanged. The five urea cycle enzymes increased with respect to the control values. Orotic acid excretion increased as well as orotidylate decarboxylase and orotate phosphoribosyltransferase, but aspartate transcarbamylase did not. The key amino acids involved in the urea and pyrimidine pathways in liver were also measured; aspartic and glutamic acids and citrulline were increased, and ornithine and arginine did not change with the higher protein intake. In general, no differences were observed between animals fed 50 and 80% protein in their diets. Protein synthesis did not increase with the increase of protein content of the diet. Stereological analysis of ultrathin sections showed that the high protein diet induced a significant increment in the volumetric density, numerical density and size of hepatocyte mitochondria. Moreover, the presence of giant mitochondria, a hundred times larger than normal, was also observed in some periportal hepatocytes of rats fed the 80% protein diet.