In a 12-week feeding study, water debittered grains of Andean lupins (Lupinus mutabilis) were tested as unique protein source in growing male and female rats. Before the phsyiological study, the lupin flour was evaluated with regard to its possible content of anti-nutritive substances, such as alkaloids, cyanogenetic glucosides, trypsin inhibitor activity and hemagutanin activity. During the feeding study, parameters such as body weight, food consumption, hemoglobin and blood cells, glucose, urea, total protein, total lipid, cholesterol, uric acid, as well as weight and histopathological examinations of some organs (liver, kidneys, heart, spleen and brain) were controlled. The lupin flour did not show physiologically significant amounts of anti-nutritive substances. Rats fed on lupin diet did not show any statistically different growth performances as compared to the casein fed control animals. Food consumption as well showed no difference between the two diets, although food utilization of casein was slightly better during the first two months. All other observed parameters gave no indication of physiological differences of the performance of the diet. Results of the 12-week feeding test did not give any indication against the use of the water extracted grains of Andean lupins in human and animal nutrition.