In 40 rats 0.5-41 days after hemiparathyroidectomy the state of the contralateral organ has been studied. Thirty-eight intact animals have been used as controls. The function of the unoperated gland is considered according to the total calcium content in blood serum. The mitotic index of parathyrocytes is calculated, nucleometric and stereologic investigation of the preparations is performed. Calcium level in the test animals' blood moderately decreases on the 2d-3d days and becomes normal in 4 days. Regeneration of the remained parathyroid gland occurs at the expense of an increased mitotic activity of the main cells from the 3d up to the 11th day and by means of hyperplasia of the nucleoli and hypertrophy of cytoplasm in epitheliocytes. The weight of the hypertrophied organ is, on the 20th, day, 77, and on the 41st day--61% of the total weight of the parathyroid glands in the control rats. Analysing dynamics of 13 quantitative parameters, it is possible to reveal certain stages of the process: the latent stage (the first 24 hours), the restorative period (the 2d--the 16th day), the normalization and stabilization of the morphometrical indices (the 19th--the 41st day).