Previous studies have demonstrated a gain-of-function of the calcium-sensing receptor (CASR) gene R990G polymorphism. In this study, activation of the R990G CASR stably transfected in HEK-293 (HEK-990G) cells compared with that of the common variant (HEK-wild-type (WT)) by increasing concentrations of CaCl₂ or calcimimetic R-568 caused significantly higher intracellular free calcium concentration ([Ca²⁺]_i) and lower Ca-EC₅₀. Moreover, the [Ca²⁺]_i oscillation percentage was higher with a larger sinusoidal pattern in HEK-990G. R-568 induced a shift of the oscillatory events from 4 to 2 mmol/l extracellular calcium concentration in HEK-990G cells and increased the sinusoidal oscillation percentage in comparison with HEK-WT. Preincubation with thapsigargin or phospholipase C inhibitors completely prevented oscillations in both cell lines, consistent with the involvement of the inositol trisphosphate pathway, while protein kinase C inhibitor prevented oscillations in HEK-WT cells only. Finally, CaCl₂ and R-568 caused a significant increase in p44/42 extracellular signaling-regulated kinase phosphorylation, with the mean Ca-EC₅₀ values being significantly lower in HEK-990G. Our findings demonstrated that the 990G allele is associated with high sensitivity to R-568, which provided new evidence for differences in CASR signaling.