The size of the parathyroid gland was evaluated at different functional levels of the gland (control: 216 glands in 54 autopsy cases, chronic renal failure: 74 glands in 21 autopsy cases, hypercalcemia: 16 glands in 15 patients with primary hyperparathyroidism). This study is based on the fact that chronic renal failure causes a hypersecretory state of parathyroid hormone (PTH), and that hypercalcemia suppresses PTH secretion. The size of the parathyroid gland was represented by the largest area cut through the hilum of the gland. Interstitial and fatty tissues were excluded from the measuring. The lower parathyroid glands are larger than the upper glands in the control. Both the upper and the lower glands enlarge with a predominance of the lower glands in size in chronic renal failure. These results suggest that the functional level of the lower glands is higher than that of the upper glands not only in the normal but in a hypersecretory state of PTH. Hypercalcemia has been shown to cause a decrease in size of the lower glands, while the upper glands scarcely decrease in size. This result indicates that the lower glands play a major role in reducing PTH secretion when PTH secretion is suppressed. It is concluded that the lower parathyroid glands play a more important role than the upper glands in the maintenance and regulation of PTH secretion.