Remineralization of early enamel lesions was studied in situ using a F chewing gum or a F-releasing device (FRD). Enamel specimens with subsurface lesions were mounted in removable lower appliances in 6 adults. A F-free dentifrice was used for all regimens. Test groups chewed five sticks/day (0.1 mg F/stick), or one FRD (0.5 mg F/day) was mounted in the midline of the appliance. The microhardness was measured after the 21-day intraoral exposure, and in vitro acid resistance testing was performed. Separate specimens were used to measure F content or changes in mineral density. Comparable values for both F gum and FRDs were higher (p > 0.05) than controls for acid resistance testing and percent remineralization. The F content for FRDs exceeded that of both F gum and controls.