Previous research in rats and humans has demonstrated that aging causes intestinal calcium malabsorption and resistance to the action of vitamin D. Here we examined whether age-associated vitamin D resistance affects induction of a classical vitamin D target gene, Cyp24a1, in the intestine and kidney of mice. Female C57BL/6J mice were obtained from the Jackson labs and after acclimation to our facility, mice were then fed with a 0.8% strontium, AIN93M based diet for 14 days to suppress renal production of 1,25 dihydroxyvitamin D3 (1,25(OH)2 D) levels. At 10 (young), 19 (mature), and 104 (old) weeks of age, mice (n=8-10 per treatment per age group) were injected with 1,25(OH)2 D (2 ng/g body weight, ip) then killed 4 h later for an organ harvest . Mucosal scrapings from the proximal (PCo) and distal colon (DCo) and kidney (Kd) samples were taken at harvest while duodenal (Dd) villus epithelial cells were isolated from the duodenum. RNA was isolated, converted to cDNA, and analyzed for Rplp0 (control gene) and Cyp24a1 mRNA by RT-PCR. Cyp24a1 mRNA was significantly induced by 1,25(OH)2 D in all segments of the intestine (Dd>PCo>DCo) and the kidney. However, there was an age dependent decline in Cyp24a1 induction in all intestinal segments and in the kidney. In all tissues, induction was significantly lower in the Old vs Young mice (Dd Villus -65.9%; DCo -51.8%; PCo -72%, Kd -53.1%). Although expression also fell from young to mature mice these changes were significant only for PCo (Dd Villus -28.5%; DCo -42.6%; PCo -55.4%, Kd -24.3%). Expression was also significantly reduced from Mature to Old mice in Kd (-38.2%), Dd Villus (-52.2%) and PCo (-37.1%) but not in DCo (-42.6%). Western blot analysis shows that VDR protein levels are not reduced by aging in the mouse small intestine. We are currently using Tag-seq analysis to explore whether the impact of aging on Cyp24a1 mRNA induction is also affecting other vitamin D regulated genes in these tissues. Future studies will examine the mechanism for this age-associated resistance to 1,25(OH)2 D action in these classical vitamin D target tissues. Supported my NIDDK award DK112365 to JCF and SC.