1. The influences of a beta-agonist, clenbuterol, on the fecal and urinary nitrogen excretion, the protein digestibility and the nitrogen balance and retention in female Wistar rats of three different weights (70, 150 and 200 g) have been studied. 2. The addition of clenbuterol to the diet did not alter the fecal nitrogen excretion, nor did it change the digestibility coefficient of the protein, irrespective of the age/weight of the animals and the amount of the beta-agonist used. 3. Clenbuterol decreased the urinary nitrogen excretion, especially in older/heavier animals and fed diets with the same dose of the agonist. 4. Nitrogen balance and retention, which, as expected, decrease as the age/weight of the animals decreases, increased significantly with the use of clenbuterol, especially in older/heavier animals. 5. From the results of this study, it can be concluded that, for the range of weight studied, the effect of this beta-agonist does not show on the absorption of proteins at the digestive level; at a metabolic level, however, it does affect the nitrogen retention although, again, to a greater extent in older/heavier animals.