Prepro-parathyroid hormone (prepro-PTH) is one of the proteins abundantly synthesised by parathyroid chief cells, yet under normal growth conditions little or no prepro-PTH can be detected in these cells. Although this may be attributed to effective co-translational translocation and proteolytic processing, proteasome-mediated degradation of PTH precursors may be important in the regulation of the levels of these precursors and hence, PTH secretion. The effects of N-acetyl-Leu-Leu-Norleucinal, N-acetyl-Leu-Leu-Methional, Carbobenzoxy-Leu-Leu-Leucinal (MG132), Carbobenzoxy-Ile-Glu-(OtBu)-Ala-Leucinal (PSI) and lactacystin on the biosynthesis and secretion of PTH were examined in dispersed bovine parathyroid cells. We demonstrate that, treatment of these cells with proteasome inhibitors caused the accumulation of prepro-PTH and pro-PTH. Compared to mock-treated cells, the processing of pro-PTH to PTH was delayed and the secretion of intact PTH decreased in proteasome inhibitor-treated cells. Relieving the inhibition of the proteasome by chasing MG132-treated cells in medium without the inhibitor led to a rapid disappearance of the accumulated prepro-PTH and the rate of PTH secretion was restored to levels comparable to those in mock-treated cells. Furthermore, over-expression of Hsp70 family of molecular chaperones was observed in proteasome inhibitor-treated cells and we show that PTH/PTH precursors interact with these molecular chaperones. These data suggest the involvement of parathyroid cell proteasomes in the quality control of PTH biosynthesis.