Bovine enamel slabs were acid softened, mounted in acrylic mandibular removable appliances, and worn by 8 male subjects. Control slabs were worn for 4 days and brushed four times daily with a fluoride-free dentifrice. Test slabs were exposed for 3 days to four times daily ingestion of 1.1 mg NaF (0.5 mg F) lozenges dissolved slowly on the tongue and then swallowed. Test slabs were worn a 4th day without exposure to the fluoride agent and brushed throughout the test period with a fluoride-free dentrifrice. Microhardness values for test slabs were significantly higher than controls following intraoral exposure and after in vitro exposure to acid. The total fluoride uptake of test slabs was significantly higher than that of controls.