There is considerable evidence which suggests that the parathyroid gland may be a target for vitamin D metabolites. To investigate the effects of 1,25-dihydroxycholecalciferol [1,25(OH)2D3] on parathyroid hormone (PTH) secretion, the serum concentrations of immunoreactive PTH (IPTH), calcium, and phosphate were measured over a 12-h period after the single iv injection of 1 μg (2.4 nmol) 1,25(OH)2D3 in five normal and nine vitamin D-deficient puppies. In the vitamin Ddeficient animals, the mean serum calcium increased significantly by 8 h, and at 12 h was approximately 1 mg/dl above the basal value. Serum phosphate concentrations showed no significant change after 1,25(OH)2D3. An acute, transient increase in serum IPTH concentration followed the injection of 1,25(OH)2D3 in most but not all of the vitamin D-deficient puppies. However, the increase was not statistically significant in the mean serum IPTH concentration for the group. At 12 h, the mean IPTH concenration had decreased significantly from the basal value by 30% (P < 0.05). Injection of 1 jug 1,25(OH)2D3 into normal puppies failed to produce a significant rise in the mean serum calcium concentration or a significant decrease in mean serum IPTH. A small, borderline, significant (P < 0.05) increase in mean serum IPTH was observed after the injection of 1,25(OH)2D3. The mean serum phosphate concentration rose significantly by 6 h (P < 0.05) and continued to rise until the end of the experiment. The response of serum IPTH to calcium infusions before and after the administration of 1,25(OH)2D3 was compared in eight vitamin D-deficient puppies. The infusions were begun 4 h after injection of 1 jug 1,25(OH)2D3 or vehicle alone. When calcium was infused rapidly (10-18 mg Ca/kg-h), serum calcium concentrations rapidly increased and serum IPTH concentrations decreased similarly under both conditions. There was a significant negative correlation between the serum calcium and serum IPTH concentrations (r = -0.872; P < 0.001). When calcium was infused slowly (6-8 mg Ca/kg-h), serum IPTH concentrations decreased minimally during the entire infusion after vehicle alone and during the initial hour of the infusion after 1,25(OH)2D3. However, during the latter portion of the infusion after 1,25(OH)2D3, serum IPTH fell, and there was a significant negative correlation between the serum calcium and serum IPTH concentrations (r = -0.564; P < 0.005). Our results indicate that 1,25(OH)2D3 has no direct inhibitory effect on PTH secretion either in hypocalcemic vitamin D-deficient or normal puppies. It may, however, increase the responsiveness of the parathyroid glands to the suppressive effect of a normal or moderately increased serum calcium concentration.