Recent studies of Baran and Milne  indicate that calcium ion inhibits 25-OH-D synthesis in rat liver homogenates in a dose-dependent fashion. Furthermore, the calcium ionophore A23187, which also increases cytosolic calcium, inhibits hepatocyte 25-OH-D production . It has been shown that 1,25-(OH)2-D inhibits 25-OH-D production . This vitamin D metabolite also increases hepatocyte cytosolic calcium levels . We have investigated the effects of the calcium-channel blocker nifedipine on serum 25-OH-D and 1,25-(OH)2-D levels in hypertensive patients undergoing RDT.