This study reports on the performance of rats fed with diets based on Canavalia brasiliensis seed proteins. The raw seeds are not a good regime, causing the animals to lose weight, exhibiting low net protein utilization (NPU) values, high nitrogen excretion and macroscopic alterations of key internal organs. Heat treatment, but not dialysis of the seed meal, had a beneficial effect on the nutritional parameters. Feeding rats with a diet containing pure lectin showed a lower digestibility and NPU, a much higher dry matter and nitrogen excretion, impaired growth rate and small-intestine enlargement in comparison with rats pair-fed on egg-white protein. In addition, lectin was detected fully active against rabbit erythrocytes in the feces of rats fed lectin-containing diets. It was suggested that the seed lectin acts as an antinutritive dietary component.