The metabolic fate of intravenously injected vitamin D₃-1,2-³H (D₃-³H) was studied in two normal individuals on chronic phenobarbital therapy. Silicic acid column chromatography of lipid-soluble plasma extracts obtained serially for 96 hr after D₃-³H injection demonstrated a decreased plasma D₃-³H half-life and increased conversion to more polar metabolites. The polar metabolites formed included several with chromatographic mobility similar to known biologically inactive vitamin D metabolites and one with chromatographic mobility identical to 25-hydroxycholecalciferol. Disappearance of this latter material was also accelerated. A child with rickets and a normal volunteer studied before and after a 2 wk course of phenobarbital therapy demonstrated similar alterations in D₃-³H metabolism. When liver microsomes from 3-wk-old Sprague-Dawley rats treated with phenobarbital were incubated with D₃-³H, polar metabolites were produced with chromatographic mobility similar to the plasma D₃-³H metabolites from phenobarbital-treated humans. Similar incubations employing 25-hydroxy-cholecalciferol-26-27-³H as the substrate also demonstrated an increased conversion to polar metabolites. The data suggest that the reported increased incidence of osteomalacia observed in patients on chronic anticonvulsant therapy may be the result of an accelerated conversion of vitamin D and its active metabolite, 25-hydroxycholecalciferol, to polar metabolites by druginduced liver microsomal enzymes.