A group of growing dogs supplemented with cholecalciferol (vit D₃) (HVitD) was studied vs. a control group (CVitD) (54,000 vs. 470 IU vit D₃/kg diet, respectively) from 3 to 21 wk of age. There were no differences in plasma levels of P_i and growth regulating hormones between groups and no signs of vit D₃ intoxication in HVitD. For the duration of the study in HVitD vs. CVitD plasma 25-hydroxycholecalciferol levels increased 30- to 75-fold; Plasma 24,25-dihydroxycholecalciferol levels increased 12- to 16-fold and were accompanied by increased renal 24-hydroxylase gene expression indicating increased renal 24-hydroxylase activity. Although the synthesis of 1,25-dihydroxycholecalciferol (1,25(OH)₂D₃) was increased in HVitD vs. CVitD (demonstrated by [³H]-1,25(OH)₂D₃ and increased renal 1-hydroxylase gene expression), plasma 1,25(OH)₂D₃ levels decreased by 40% as a result of the even more increased metabolic clearance of 1,25(OH)₂D₃ (demonstrated by [³H]-1,25(OH)₂D₃ and increased gene expression of intestinal and renal 24-hydroxylase). A shift of the Ca setpoint for parathyroid hormone to the left indicated increased sensitivity of the chief-cells. Effective counterbalance was provided by hypoparathyroidism, hypercalcitoninism and by the key-regulator 24-hydroxylase preventing the development of vitamin D₃ toxicosis.