Male weanling Sprague-Dawley rats were administered phenobarbital or diphenylhydantoin (phenytoin, dilantin, DPH), or a combination of the two drugs, while receiving a high calcium, low phosphorus vitamin D-deficient diet. A Line Test evaluation for calcification of the distal end of the radius was performed to determine the presence of rickets and the healing of the disease. After a period of 16 days, rickets had developed in a uniform manner in all groups regardless of drug treatment. Four I.U. of vitamin D3 were then administered to each animal and they were sacrificed either 3 or 9 days later. Three days after the administration of D3, bone calcification had started in all groups, with the greatest response being observed in the control group. Nine days after D3 administration, bone calcification was clearly impaired in PB and PB-DPH treated animals and slightly impaired in the DPH treated group when compared to controls. Weight gain was also impaired after PB and combined PB-DPH treatments. Based on these observations, it is suggested that anticonvulsant drugs, and in particular PB, inhibit, in a yet undetermined fashion, the normal calcification of the growth cartilage which accompanies a single physiological dose of cholecalciferol in the rat.