Weanling rats were fed diets with normal (1%) or low (0.08% or 0.02%, respectively) Ca content for 28 days prior to sacrifice. The total volume of the parathyroids was estimated from serial sections. Volume density of secretory cells was calculated according to conventional stereological techniques, whereas cell number and cell size were estimated by the dissector method. Compared with controls the animals of the experimental groups developed moderate and severe hypocalcemia and their parathyroids were enlarged with a proportional growth of parenchyma and interstitium. Related to the body weight, secretory cell volume was highest in animals with severe hypocalcemia. In the enlarged glands the size of parathyroid secretory cells was increased by 30-40%, whereas total cell number was unaltered. Thus, the increased parathyroid size was due to cell hypertrophy rather than hyperplasia.