ABSTRACT. Fractional intestinal 47Ca calcium absorption (α) in 12 epileptic outpatients receiving chronic high-dose anticonvulsant therapy was reduced (p>0.05) compared to 12 matched normal controls. Six of the epileptics were treated orally with 0.5 μg of 1,25-dihydroxycholecalciferol (1,25-DHCC) per day and six with 10 μg of 25-hydroxycholecalciferol (25-HCC) per day for 10 days. The α was determined before and after treatment and compared with the effect of 0.5 μg of 1,25-DHCC per day given for 10 days to 6 controls. An increase of the same order in α was found in all groups (p>0.05). No changes were observed in the serum levels of calcium, phosphorus, alkaline phosphatase or iPTH during treatment. Urinary calcium excretion was low in the epileptic patients and rose during treatment. The investigation demonstrates that the sensitivity of the intestine to 1,25-DHCC is normal in epileptic patients on anticonvulsant therapy and that 1,25-DHCC and 25- HCC in the given doses had an equal effect on the reduced intestinal calcium absorption.