1. In vitamin D-deficient chicks both vitamin D3 and 1α-hydroxycholecalciferol markedly decrease renal 1-hydroxylase activity and induce 24-hydroxylase activity.
2. Actinomycin D abolishes both effects.
3. These results are consistent with feedback regulation of vitamin D3 metabolism by a direct nuclear action of the vitamin or its metabolites on the kidney cells.