Recently, a calcium-sensing receptor (CaSR) from bovine parathyroid tissue was cloned, and Northern blot analysis revealed expression of CaSR mRNA in several tissues other than the parathyroid gland. In the present study, we fractionated bone cells from 1-to 2-day-old rat calvaria by digestion with collagenase and EDTA and examined the expression of CaSR mRNA in cells of the various fractions by reverse transcription polymerase chain reaction (RT-PCR) analysis and in situ hybridization histochemistry. Fraction I cells had an elongate shape, and cells from fraction II through IV had a stellate and polygonal shape. Cells from fraction VI were small and stellate-shaped, and possessed a large number of long cytoplasmic processes. Fraction m cells expressed alkaline phosphatase (ALPase) activity and osteocalcin (Osc) mRNA, suggesting them to be osteoblasts. On the other hand, only fraction vl cells expressed CaSR mRNA ; and they displayed no ALPase activity or Osc mRNA. Furthermore, fraction VI cells responded to the elevated extracellular calcium with a rapid elevation of their cytosolic calcium. The phenomenon was independent of membrane voltage and insensitive to organic calcium channel modulators, such as BAY K 8644, nifedipine, and nicardipine. These findings strongly suggest that rat mature osteocytes express a CaSR that responds to elevated extracellular calcium.