To contribute to our understanding of nitrogen metabolism in the developing chick we have studied in liver, intestine and yolk sac membrane the ontogeny of both aspartate- and alanine transaminases, glutamate dehydrogenase, adenylate deaminase, glutamine synthetase and xanthine dehydrogenase activities. Liver enzyme activities were much higher than those of the same enzymes in intestine and yolk sac membrane, the latter having the lowest activities. In the liver, both alanine transaminase and glutamate dehydrogenase increased their activity just before hatching, xanthine dehydrogenase and glutamine synthetase develop their highest activity just after hatching, while aspartate transaminase and adenylate deaminase attained the highest levels just with adulthood. From the pattern of enzyme activity in yolk sac membrane and intestine it can be inferred that after hatching, the amino-acid metabolism in these tissues is considerably enhanced, with higher production of ammonia from amino acids, as indicated by the rise in adenylate deaminase, as well as increased potentiality in production of both alanine and glutamine. It can be concluded that hatching coincides with a deep change of pace in amino-acid metabolism in the organs studied fully comparable with that observed in Mammals at the end of lactation, with the difference that the adaptation to the new diet in the case of the chick is much more sudden than weaning is for the rat.