Wistar rats of 60.5 +/- 5.0 g fasted for 24 h were injected intraperitoneally with 10 microCi of [14C]glycine. One to two hours after injection the rats were fed a diet containing 10.53 +/- 0.75% protein provided by dry beans or casein, or a protein-free diet and submitted to a 4-d nitrogen balance. Radioactivity in the feces of rats fed casein, cooked beans and raw beans was roughly 2, 5 and 10 times greater, respectively, than in the feces of those fed the protein-free diet. Apparent protein digestibility showed a strong negative linear correlation (r = -0.9805, P less than or equal to 0.01) with radioactivity in the feces. Positive correlation (P less than or equal to 0.01) was demonstrated between radioactivity and either total carbon or total nitrogen in the feces of rats injected with [14C]glycine. Mean value for the radioactivity in the urine of rats fed the different diets did not differ significantly (P less than or equal to 0.05). Endogenous nitrogen excretion of rats on bean diets was estimated by the ratio of total endogenous N to marker N, based on the protein-free diet. The results indicated that rats fed bean-containing diets excreted significantly more endogenous nitrogen than those fed the casein diet, even though the casein diet had stimulated twice as much endogenous excretion than the protein-free diet. As a consequence, apparent digestibility and biological value of bean protein are generally underestimated, although the "real" biological value was not affected by the endogenous nitrogen excretion of the rat.