In order to investigate the in vivo mechanisms of target gene activation by vitamin D₃ analogs, we compared the effects of two vitamin D₃ analogs, 22-oxa-1α,25-(OH)₂D₃ (OCT) and 2β- (3-hydroxypropoxy) −1α,25-(OH)₂D₃ (ED-71) with that of 1α,25-(OH)₂D₃ on 1α,25-(OH)₂D₃ −24-hydroxylase[24(OH)ase] mRNA expression in the kidney and intestine of normal rats. In these experiments, all three compounds induced 24(OH)ase mRNA, but the time course of induction for each respective treatment was clearly different. OCT caused the most rapid onset of increased 24(OH)ase mRNA expression and its subsequent return to pre-injection levels. In marked contrast, ED-71 was the slowest to increase expression which was prolonged over that observed with the other compounds tested. These differences probably relate to the pharmacokinetic properties of these analogs, which are mainly generated by the affinity of analogs for the vitamin D-binding protein(DBP).