The effects of two models of chronic hypercalcemia on renal acid-base metabolism were studied in rats. In the first series of experiments, rats were rendered hyperparathyroid by the autologous grafting of 20 to 24 parathyroid glands into a single recipient. Hypercalcemia (5.48 +/- 0.03 mEq/liter in high PTH animals, 4.96 +/- 0.06 mEq/liter in pair-fed controls, P less than 0.001) occurred as did metabolic alkalosis (plasma total carbon dioxide 25.44 +/- 0.47 mEq/liter vs. 23.84 +/- 0.57 in controls, P less than 0.05). The rise in total carbon dioxide was in part a renal tubular effect since urine pH was lower (6.77 +/- 0.04 vs. 6.95 +/- 0.04, day 5, P less than 0.01) bicarbonaturia less (165 +/- 26 vs. 283 +/- 28 mumoles/24 hr, day 5, P less than 0.01) and titratable acid (TA) excretion increased (164 +/- 43.4 vs. 48.2 +/- 2.53 mEq/24 hr, day 5, P less than 0.01) in hyperparathyroid animals vs. pair-fed controls. To test the specific role of hypercalcemia versus PTH in this effect, normoparathyroid animals were treated with 1.25 (OH)2 vitamin D3 or SHAM injected, Urinary cAMP was reduced in these animals (0.030 +/- 0.004 mumoles/8 hr) compared to hyperparathyroid rats (0.055 +/- 0.01 mumoles/8 hr P less than 0.05) suggesting differences i PTH levels. Hypercalcemia occurred in 1,25(OH)2 vitamin D treated animals as did increased plasma total carbon dioxide and urinary TA while urinary bicarbonate excretion and urinary pH were reduced. Because hypercalcemia was associated with elevated total carbon dioxide in both models, it is proposed that chronic hypercalcemia stimulated renal acid excretion and in a sustained manner results in metabolic alkalosis, at least in part, on a renal basis.