The concentration of calcium was measured in the aorta, heart, and kidney of uremic rats treated with 100 ng/kg/day 1,25-dihydroxyvitamin D3 (1,25 D3) or 60 mg/kg/day diltiazem for 12 weeks. The concentration of calcium was increased in the aorta, heart, and kidney of uremic rats, and was further increased by administration of 1,25 D3. The 1,25 D3-induced increase in calcium in the aorta was inhibited by diltiazem, but this effect was not accompanied by a decrease in serum calcium x phosphate products. Diltiazem had no effect on the 1,25 D3-induced increase of calcium in the heart and kidney. Thus, in uremia 1,25 D3 may promote the calcification of the aorta; calcium antagonists may protect against calcification without a reduction in serum calcium x phosphate products.