To quantify the effect of rice cereal on nitrogen balance and fecal nitrogen excretion, and the contribution of endogenous nitrogen sources to fecal nitrogen output, nine infants aged 3-5 mo received [15N]glycine in all feedings. Samples of urine and feces were obtained daily and analyzed for nitrogen and 15N. On days 1-7 the infants received only formula, and a complete urine and fecal collection was made on days 6-7. From days 8 to 12 the formula was supplemented with rice cereal (133.3 g/L, or 4 g/30 mL formula) and a second complete urine and fecal collection was made on days 11-12. Cereal did not alter fecal nitrogen output or the calculated contribution (45%) of endogenous nitrogen to fecal nitrogen. Cereal did increase nitrogen retention and lower the urinary excretion of the 15N dose (24% of dose). The calculated true digestibility of the rice cereal protein was > 95%. Our results indicate that infants aged 4 mo, in contrast with younger infants, are able to digest and absorb cereal. The addition of cereal to the diet does not lead to increased fecal protein losses.