The concentrations of fluoride in the gingival crevicular fluid (GCF) of dog canines and carnassials were determined as a function of plasma fluoride levels using the filter paper strip technique. These were compared to the simultaneous concentrations of 131I 99mTc which were assumed to be markers for evaporative water loss of GCF. Plasma fluoride levels ranged from 15 to 300 μmol/l. The GCF to plasma fluoride concentration ratios at all plasma fluoride levels were consistently greater than unity. This was also true for the 131I and 99mTc GCF to plasma ratios. When the GCF to plasma fluoride ratios were factored by the radiotracer ratios, the values closely approximated unity. Thus, 131I and 99mTc appear to be valid markers for evaporative water loss of GCF. It is concluded that GCF fluoride levels are nearly identical to those of plasma over a wide range of plasma fluoride concentrations and that GCF is a source of fluoride for the oral environment.