From the work reported it can be seen that the in vivo neutron activation analysis method can be used to measure nitrogen in vivo in small animals (mass about 5 kg) with a reproducibility of +or-7% (1 SD). Measurements of nitrogen in minced beef were consistent with the chemical analysis method. For live animals the statistical error was low enough to determine the quantity of nitrogen to better than 10%. To improve this method one should use more detectors to increase the statistical accuracy without increasing the dose or time.