The parenchymal cell mass in parathyroid glands from 368 autopsy cases without apparent parathyroid or kidney disease was investigated. By measuring the glandular density with a density gradient technique and the glandular weight, the parenchymal to fat cell ratio and the weight of the parenchymal cell mass could be determined. The mean parathyroid parenchymal cell weight was calculated to be 22 mg and the maximal normal parenchymal cell weight 39 mg. The glandular weight was evidently dependent on the body constitution, as the fat tissue content of the glands was related to the amount of fat tissue in the body as a whole. The parenchymal cell weight was much less variable. Also, the amount of fat tissue in the parathyroid glands was positively correlated to the size of glands. Thus, the amount of fat cells within parathyroid glands seems less adequate as a histological criterion and the parenchymal cell weight should be of greater value in the histological diagnosis that the weight of the whole gland. The age-and-sex-related variations of the parenchymal cell mass were slight and therefore a value of 39 mg may be considered an appropriate upper limit of the weight of this mass in parathyroid glands of adult persons.