Suppression of PTH secretion by extracellular calcium is mediated by a plasma membrane calcium receptor (CaR). However, primary cultures of bovine parathyroid cells are known to quickly lose their responsiveness to extracellular calcium. The present study was designed to determine if the loss of calcium responsiveness is due to changes in CaR expression. In primary monolayer cultures of parathyroid cells, calcium-mediated suppression of PTH was still evident after 24 hours in culture but was completely absent after 6 days. This was preceded by a 75% drop in CaR mRNA content within 24 hours. CaR mRNA levels remained low for the 6-day culture. Earlier time points, examined in parathyroid cell suspensions, showed a 70% drop in CaR mRNA by 4 hours after collagenase-dispersion of the glands and an 85% drop after 24 hours. The decreased expression of CaR mRNA was not influenced by altering medium serum, calcium, or 1,25-dihydroxyvitamin D₃. Our results indicate that the loss of responsiveness of cultured parathyroid cells to calcium is due to decreased CaR mRNA and, presumably, CaR protein expression.