growth with a variety of target organs, including the skeleton, intestine, and kidney (7). 24,25(OH) 2 D 3 is a biologically active metabolite mainly directed to the skeleton (6, 56) but also with putative actions at the intestine (39,40,59,60).The rate of renal synthesis of 1,25(OH) 2 D 3 is directly responsive to plasma levels of P i , growth hormone (GH), insulin-like growth factor I (IGF-I), parathyroid hormone (PTH), and calcitonin (CT; see Refs. 24,36,and 41). Regulatory feedback on 1␣-hydroxylase is provided by 1,25(OH) 2 D 3 by induction of 24-hydroxylase activity and thus conversion of 1,25(OH) 2 D 3 into less biologically active metabolites in its target tissues, including intestine, kidney, and bone (7, 52). In the kidney, 24-hydroxylase activity is enhanced by 1,25(OH) 2 D 3 and downregulated by PTH (48, 62, 63), whereas in the intestine, 24-hydroxylase is enhanced by 1,25(OH) 2 D 3 and downregulated by CT (3).The period of rapid growth is a formidable challenge for vitamin D 3 metabolism in preserving skeletal mineralization. There are few investigations in young intact animals that have studied the hormonal regulation of excessive vitamin D 3 with respect to the activity of 1␣-hydroxylase and 24-hydroxylase (4, 47, 57, 58). However, these studies confined their measurements to single-moment observations, possibly because of technical limitations. Therefore, there is insufficient knowledge concerning the timedependent changes of vitamin D 3 metabolism during elevated dietary vitamin D 3 intake. Obtaining insight into the complexity of vitamin D 3 homeostasis in relation to its regulating hormones and enzymes requires large research animals for long-term studies on the effect of dietary vitamin D 3 supplementation. Dogs are of adequate size to allow for simultaneous and sequential sampling of blood and tissue material during the rapid growth period. Additional advantages are complete dependence on the dietary intake of vitamin D 3 (29) and thus easy regulation of the vitamin D 3 status without interpretation problems caused by seasonal variation of plasma vitamin D 3 metabolite concentrations.