The influence of diets containing faba bean or casein as sources of protein were studied in rats at two stages of development. A significant impairment of growth rate, carcass, liver and skeletal muscle were found in young and adult rats fed for a period of 10 days on raw legume. Urinary urea output and the activities of three amino acid degrading enzymes: arginase, alanine aminotransferase and arginine succinate synthetase were all affected by the dietary protein and the stage of development. Urinary creatinine excretion was higher in the adult rats, while serum cholesterol was slightly increased in the young ones. Changes in plasma zinc may be attributed to a reduced zinc bioavailability to rats from the faba bean diet. Other biochemical parameters measured (glucose, triglycerides and plasma proteins) remained unchanged in all the experimental groups. Liver DNA and RNA content (mg/g tissue) decreased with age in both dietary groups, which were accompanied by an increase in tissue size. Furthermore, liver RNA concentration (primarily a measure of protein synthesis capacity) was enhanced in the adult legume fed rats. In this context, it is suggested that other organs (particularly muscle, with lower amino acid requirements for protein synthesis as a consequence of the stunting of growth) could contribute to increase the amino acid supply to liver in the animals fed on the faba bean diet.