Background/Methods: We applied a novel method for studying endothelial cells (EC) by using autoradiography of cells labeled by <sup>3</sup>H-thymidine: photo emulsion was administered into the vascular bed. In the flat transparent organ, this method allows to compare the mitotic activity of endothelium (MAE) depending on vessel diameter and animal age. Results: The number of the labeled ECs in animals was found to be almost equal in all vessels at the same age. The amount of labeled vessels and the density of the labeled nuclei increase towards near-capillary vessels of a diameter ≤10 µm. With age, ÐœÐE decreases in all categories of vessels. However, in 12-day-old animals, ÐœÐE temporarily increases. It was noted that high ÐœÐE in the bed of these rats precedes or coincides with the period of accelerated weight gain of the digestive system supplied by an increase in organ blood flow. Conclusions: By using the endothelial autoradiography method that we developed, we obtained the following evidence: (1) the number of ECs in the synthetic phase of the mitotic cycle in mesenterial vessels is approximately identical in animals of the same age, and (2) this number decreases with age, except during the period of accelerated organ growth, when MAE increases.