AbstractWe investigated the stimulation of creatine kinase (CK) and ornithine decarboxylase (ODC) by 1,25‐dihydroxyvitamin D3[1,25(OH)2D3] and 24R,25‐dihydroxyvitamin D3[24R,25(OH)2D3] in doses ranging from 1.625 to 6500 pmol in 4‐week‐old vitamin D‐deficient chicks. Enzyme activities were monitored for 72 h. 1,25(OH)2D3but not 24R,25(OH)2D3enhanced the activity of ODC in duodenum and bone. The time course of ODC activity in bone was biphasic, with an increase after 1 h and a higher peak after 24 h. Diaphyses and epiphyses responded equally well after a dose of 6500 pmol. The kidney, liver, and lung showed 1.5–3.8‐fold increase in CK activity following 1,25(OH)2D3, reaching a maximum between 3–5 h. However, sustained stimulation of CK activity could still be demonstrated after 72 h, and the 48‐h levels in the lung even exceeded the 5‐h values. No change of activity of either enzyme was noted in heart and brain after application of 1,25(OH)2D3. There was no coincidence of stimulation of ODC and CK by 1,25(OH)2D3in the same tissue, and the dose‐responsiveness of both enzymes differed considerably. Near maximum activities of ODC were achieved with 19.5 pmol 1,25(OH)2D3in duodenum and pancreas, while maximum responses of CK occurred in the liver at 195 pmol and in lung and kidney at 6500 pmol. 24R,25(OH)2D3failed to produce any consistent effects of either enzyme in all tissues examined. These results, particularly the lack of response to 24R,25(OH)2D3, are different from t