The influence of low calcium diets (0.08%) with or without a deficiency in vitamin D (D) on the parathyroid function was studied in two groups of six dogs. The animals were first studied on a normal diet and then again after 3 weeks of the experimental diet. Blood tests, urine tests and a functional evaluation of the parathyroid glands via i.v. infusions of CaCl2 and NaEDTA were performed on both occasions. PTH was measured with an intact hormone assay (I) and with a carboxylterminal assay (C). Since similar results were observed on the D deficient and D normal diets at 3 weeks, data were combined for final analysis. We observed an increase in fasting serum PTH (I, 3.2 +/- 2.0 vs. 4.3 +/- 3.3 pmol/l, P less than 0.05; C, 23.4 +/- 13.9 vs. 30.7 +/- 15.5 pmol/l, P less than 0.005) and in stimulated serum PTH (I, 11.7 +/- 2.7 vs. 18.3 +/- 4.5 pmol/l, P less than 0.0005; C, 67.7 +/- 22 vs. 90.4 +/- 31.1 pmol/l, P less than 0.0005) after 3 weeks of a low calcium diet. Fasting ionized calcium concentrations (1.36 +/- 0.03 vs. 1.36 +/- 0.02 mmol/l), 25(OH)D concentrations (94.8 +/- 28 vs. 86.7 +/- 23.1 nmol/l) and 1,25(OH)2D concentrations (101.1 +/- 19.3 vs. 110.9 +/- 27.6 pmol/l) did not change. The increase in parathyroid function measured with I (60.4 +/- 39%) was greater than that measured with C (33.7 +/- 14.2%, P less than 0.05) and the ratio of maximum carboxylterminal PTH to maximum intact PTH decreased from 5.98 +/- 2.17 to 4.95 +/- 1.21 (P less than 0.05) at 3 weeks. These results suggest that reduced catabolism of intact PTH was involved in the increased parathyroid function. The stimulus responsible for the increased parathyroid function remains to be identified.