EXPERIMENTALLY (1, 2) and clinically (3) the orthodox action of vitamin D in the absence of the parathyroids is characterized by effects similar to those of parathyroid hormone administration. Harrison and Harrison (4) have shown, however, that in rachitic dogs, vitamin D administration, in contrast to the effect of giving parathyroid extract, results in lessening phosphaturia. More recently, Pincus and Gittleman (5) have presented data indicating that even small doses of vitamin D included in the formulae of newborn infants, especially when these are of high phosphate content, result in lowering of serum calcium and elevation of serum inorganic phosphate concentrations. These latter, paradoxical effects of the vitamin could indicate that under certain circumstances, particularly situations of physiologically increased parathyroid activity, the vitamin has a direct inhibitory action on parathyroid hormone release. The studies here reported were undertaken to investigate this possibility using rats fed a p...