As most of the studies on the regulation of renal 25-hydroxycholecalciferol-1-hydroxylase (25-HCC-1-hydroxylase) activity have been done in marginally-vitamin D-deficient animals and as it is known that vitamin D administration suppresses the specific activity of the 25-HCC-1-hydroxylase, it was decided to study the effect of dietary calcium on the activity of 25-HCC-1-hydroxylase and on Ca absorption in vitamin D-replete chicks. 2. Chicks, 10 d old, were given diets differing in their Ca contents (65 nmol cholecalciferol/kg diet) for 10 d and the activity of 25-HCC-1-hydroxylase in kidney homogenates, Ca absorption from the duodenum, Ca-binding protein (CaBP) activity in the duodenal mucosa and plasma Ca and phosphate concentrations were all determined. 3. The CaBP activity and the efficiency of Ca absorption both decreased with increasing dietary intake of Ca. Ca absorption and CaBP activity were significantly correlated (r 0-995, P less than 0-01). 4. The activity of 25-HCC-1-hydroxylase decreased as the dietary level of Ca increased and was significantly correlated with Ca absorption (r 0-900, P less than 0-05). The plasma Ca concentration and the activity of 25-HCC-1-hydroxylase were inversely related (r-0-940, P less than 0-01). 5. It is concluded that in the vitamin D-replete chick the efficiency of duodenal Ca absorption is regulated by the renal 25-HCC-1-hydroxylase activity via production of 1,25-dihydroxycholecalciferol and CaBP synthesis.
