Vitamin D-24-hydroxylase (CYP24) is one of the enzymes responsible for vitamin D metabolism. CYP24 catalyzes the conversion of 25-hydroxyvitamin D-3 [25(OH)D-3] to 24,25-dihydroxyvitamin D-3 [24,25(OH)(2)D-3] in the kidney. CYP24 is also involved in the breakdown of 1alpha,25-dihydroxyvitamin D-3 [1alpha,25(OH)(2)D-3], the active form of vitamin D3. In this study, we generated transgenic (Tg) rats constitutively expressing CYP24 gene to investigate the biological role of CYP24 in vivo. Surprisingly, the Tg rats showed a significantly low level of plasma 24,25(OH)(2)D-3. Furthermore, the Tg rats developed albuminuria and hyperlipidemia shortly after weaning. The plasma lipid profile revealed that all lipoprotein fractions were elevated in the Tg rats. Also, the Tg rats showed atherosclerotic lesions in the aorta, which greatly progressed with high-fat and high-cholesterol feeding. These unexpected results suggest that CYP24 is involved in functions other than the regulation of vitamin D metabolism. (C) 2002 Elsevier Science (USA). All rights reserved.