To investigate whether the effect of the research calcimimetic AMG 641 and calcitriol on CaSR and VDR expression could be separated from their ability to reduce parathyroid cell proliferation, five-sixth nephrectomized (5/6 Nx) rats received vehicle, AMG641, calcitriol or AMG641+calcitriol either daily for 13 days (long-term protocol) or in a single dose (short-term protocol). In the long-term protocol, AMG641, calcitriol and their combination significantly reduced the percent of proliferating parathyroid cells. Proliferation was uncontrolled in the short-term protocol. A significant increase in CaSRmRNA (% vs -Actin) was detected in rats treated with both calcitriol (1.60±0.30) and AMG641 (1.66±0.25) for 13 days (P=0.01 vs 5/6 Nx+vehicle, 0.89±0.09); and there was a further increase when both drugs were administered simultaneously (2.46±0.33). In the short-term protocol only rats receiving AMG641 alone (2.01±0.33, P<0.001) increased expression of CaSRmRNA whereas the combination (1.81±0.20) produced no additional benefit. AMG641 also increased CaSRmRNA expression in vitro. Changes in VDRmRNA paralleled those of CaSRmRNA. In the long-term treatment, both AMG641 (0.87±0.14) and calcitriol (0.99 ± 0.12) increased VDRmRNA (P<0.05 vs 5/6 Nx+vehicle, 0.49±0.10) and the increase was more accentuated when the drugs were combined (1.49±0.45). In the short-term protocol, only treatment with AMG641, alone (1.52±0.41) or combined with calcitriol (1.86±0.24), increased VDRmRNA. In conclusion, our results demonstrate an acute increase in CaSRmRNA and VDRmRNA in the parathyroid glands of uremic rats treated with AMG641 in which cell proliferation was uncontrolled, thus supporting a direct effect of calcimimetics on CaSR and VDR expression by hyperplastic parathyroid cells.