Secondary hyperparathyroidism is characterized by an increase in parathyroid (PT) cell number, and parathyroid hormone (PTH) synthesis and secretion. It is still unknown as to what stimuli regulate PT cell proliferation and how they do this. We have studied rats with dietary-induced secondary hyper- and hypoparathyroidism, rats given 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) and rats after 5/6 nephrectomy for the presence of PT cell proliferation and apoptosis. PT cell proliferation has been measured by staining for proliferating cell nuclear antigen (PCNA) and apoptosis by in situ detection of nuclear DNA fragmentation and correlated with serum biochemistry and PTH mRNA levels. A low calcium diet led to increased levels of PTH mRNA and a 10-fold increase in PT cell proliferation. A low phosphate diet led to decreased levels of PTH mRNA and the complete absence of PT cell proliferation. 1,25 (OH)2D3 (25 pmol/d x 3) led to a decrease in PTH mRNA levels and unlike the hypophosphatemic rats there was no decrease in cell proliferation. There were no cells undergoing apoptosis in any of the experimental conditions. The secondary hyperparathyroidism of 5/6 nephrectomized rats was characterized by an increase in PTH mRNA levels and PT cell proliferation which were both markedly decreased by a low phosphate diet. The number of PCNA positive cells was increased by a high phosphate diet. Therefore hypocalcemia, hyperphosphatemia and uremia lead to PT cell proliferation, and hypophosphatemia completely abolishes this effect. Injected 1,25 (OH)2D3 had no effect. These findings emphasize the importance of a normal phosphate and calcium in the prevention of PT cell hyperplasia.