Fluoride release from a glass-ionomer cement was measured in vivo during an 8-day period. Maxillary arcylic resin appliances, each carrying four glass-ionomer cement specimens, were worn by four subjects at night. Unstimulated whole saliva samples were collected before the subjects retired to bed (pretreatment samples) and in the morning (posttreatment samples). The average fluoride concentration and saliva flow of pretreatment samples were 31 ± 4 ng/mL and 0.5 ± 0.1 mL/min, respectively. In all subjects there was a statistically significant posttreatment increase (.005 P .025) in salivary fluoride concentration. The release of fluoride was nearly constant during the test period.