Systemic fluoride induces osteoclastogenesis in C3H/HeJ (C3H) mice with increased serum iPTH, sRANKL, and TRAP5b along with reduced levels of OPG; whereas, in C57BL/6J (B6) mice fluoride has anabolic actions (Yan et al 2007). In humans pseudo-hyperparathyroidism is sometimes seen with skeletal fluorosis. Early events of fluoride’s actions on the parathyroid gland are not well understood. Objective: This study investigated fluoride’s effects on iPTH secretion and its underlying mechanism. Methods: In vitro: Thryo-parathyroid complexes (TPC) from 5-6 week old C3H (n=18) and B6 (n=18) male mice were dissected and dispersed. TPC were cultured in medium pre-optimized for calcium, phosphorus, and magnesium levels equivalent to mouse serum levels. TPC were treated with 0, 250 or 500µM [F-] for 24hrs and secreted iPTH levels determined by ELISA. HEK293T-cloned cells (containing luciferase reporter) were transfected with human calcium sensing receptor (CASR) or orphan GPCRs followed by 0 or 1mM fluoride treatment (16hrs) to determine ?-arrestin recruitment. In vivo: 5-6 week old C3H (n=78) and B6 (n=78) male mice were gavaged with distilled or fluoride water (1µg [F–]/g-body weight). Serum iPTH, calcium, phosphorus, magnesium, and fluoride were determined at 0, 3, 6, 12, or 24hrs. Results: A dose-dependent decrease in secreted iPTH was seen in vitro for C3H (p<0.005) and B6 (p<0.005) at 500µM [F–] exposure. Single gavage led to an increase in serum fluoride peaking at 0.5hr. iPTH also peaked at 0.5hr in both C3H (p=0.002) and B6 (p=0.01) followed by a decrease in C3H at 24hrs (p=0.002) returning to baseline at 48hrs. At 12hrs, there was an increase in iPTH in B6 (p<0.001) which returned to baseline at 24hrs. Fluoride treatment did not alter ?-arrestin recruitment by CASR. GPR111, GPR142 and GPRC5D demonstrated increased ?-arrestin recruitment. Conclusion: Fluoride modulates iPTH secretion in vitro and in vivo. However, Fluoride’s action on the parathyroid gland is not mediated through CASR. While fluoride’s effects, in vitro, were equivalent between the two mouse strains, early strain-dependent effect on iPTH secretion was observed in vivo. Difference in fluoride-mediated gene expression in C3H and B6 suggests an underlying difference in physiologic handling of fluoride by the two strains.