We examined the effects of chronic anticonvulsant drug administration on the serum concentrations of dihydroxyvitamin D metabolites and the response of these metabolite levels to short-term treatment with pharmacologic doses of vitamin D2. Twelve patients maintained on chronic combined diphenylhydantoin and phenobarbital therapy were studied before and after the administration of vitamin D2, 75,000 units/week for 6 weeks. Prior to vitamin D2 administration, the patient group demonstrated decreased serum calcium (P less than 0.01) and increased serum iPTH (P less than 0.02) concentrations relative to 18 matched controls. Serum 25OHD and 24,25(OH)2D concentrations in the patient group were reduced by 38% and 75% (P less than 0.001), while serum 1,25(OH)2D concentration was increased by 27% (P less than 0.05) relative to control values. After vitamin D administration serum calcium and iPTH concentrations in the patient group were restored to values that were not significantly different from control levels. Serum 25OHD and 24,25(OH)2D concentrations were increased by 4.6- and 6.3-fold, respectively, to supranormal levels. Serum 1,25(OH)2D concentration exhibited an unexpected further 30% increase over pretreatment values, resulting in a return of the serum 1,25(OH)2D/24,25(OH)2D concentration ratio to a normal value. These data indicate that in patients treated chronically with anticonvulsant drugs, serum 25OHD concentration responds appropriately to vitamin D2 administration, but regulation of renal dihydroxy metabolite formation may be altered.