The biotransformation of flecainide to serum fluoride after the oral administration of 100 mg to six healthy subjects was studied. Fluoride, flecainide acetate, calcium and alkaline phosphatase serum levels were determined at 0, 3, 4.5 and 6 hours after administration. Higher mean serum concentrations for fluoride and alkaline phosphatase (P less than 0.01, P less than 0.005) and lower mean calcium levels (P less than 0.05) were reached at 4.5 hours. The flecainide acetate serum levels ranged from 108 to 261 ng/ml. The results were analyzed for statistical significance by single factor analysis of variance with repeated measures. The Spearman rank correlation coefficient between flecainide acetate and fluoride serum increases was calculated. Our results would suggest that fluorine-containing flecainide could be biotransformed to yield some ionic fluoride which contribute to the daily fluoride intake.