The effect of orotic acid on pulmonary regeneration has been studied in mature rats after left-sided pneumonectomy. A complex morphometric analysis of the pulmonary parenchyma during the process of compensatory-hypertrophic rearrangement has been carried out; mitotic activity of cells in the interalveolar septae, peculiarities of DNA synthesis and those of enzymatic status in lymphocytes of the peripheral blood have been determined. As the investigations have demonstrated, the administration of orotic acid during the postoperative period contributes to a more complete regeneration of the lung volume, alveolar surface, results in formation of new alveoli, prevents the development of morphologic long-term decompensation following pneumonectomy. Activation of the alveolar cells proliferation and increased DNA synthesis are the main stimulating effects of orotic acid. Certain correlative connections are revealed between the course of pulmonary regenerative processes and the enzymic status of circulating lymphocytes.