The present paper shows the effect of egg yolk on chick kidney ornithine transcarbamylase (OTC) activity. A 90% egg yolk diet resulted in an increase of about 2.5-fold in kidney OTC activity. Enzyme activity increased gradually during incubation and after hatching and then decreased in accordance with disappearance of egg yolk from the residual yolk sac. Maximum activity - 10-fold that of the 14th day embryo - was observed between Days 4 and 7 in chicks fed the control diet. This maximum was shifted to between Days 14 and 21 and to a higher level by feeding a 90% egg yolk diet. Feeding the egg yolk diet to 14- or 21-day-old chicks that had received commercial chick starter from Day 2 also caused an increase of OTC activity. It was suggested that egg yolk or an ingredient(s) induces chick kidney OTC and that utilization of egg yolk promotes the formation of OTC in chick kidney during embryonic development and during subsequent growth of the chick.