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The effect of atriopeptide AP-II and its 6-amino-acid acyclic fragment (7-12) AP-H-6-OH on proliferative processes in the myocardium of 5-day-old albino rats is examined 24 h after a single administration (200 nmol/kg intraperitoneally) by autoradiography with 3H-thymidine and by analysis of mitotic regime. A significant decrease in the amount of DNA-synthesizing ceils in the right atrium, right ventricle, and interventricular septum is recorded after administration of AP-II. Changes in DNA synthesis are attended by significant changes in the amount of cells in diffei'ent phases of mitosis, indicating a decrease in the number of mitotic cells and a metaphasal delay. Administration of AP-H-6-OH causes no significant changes in DNA synthesis in the myocardium at the early stage of postnatal ontogenesis.Key Words: atriopeptides; proliferation; myocardium; postnatal ontogenesis Atriopeptides participate not only in the regulation of fluid-and-electrolyte balance, but also in the maintenance of structural homeostasis. Proliferative effects of atriopeptides on mesangial [7], endothelial [9], and vascular smooth muscle cells [13] have been demonstrated in vitro. Atriopeptides were shown to influence physiological regeneration of renal epithelia and cortex in vivo [1].The presence of atriopeptides in mammalian myocardium at the early stages of heart development [14] and the effect on the proliferation of cultured chick embryo cardiomyocytes [10] suggest that atriopeptides are involved in cardiac morphogenesis.The aim of this study was to examine the effect of atriopeptides on cell reproduction in albino rat myocardium at the early stages of postnatal development. Central Research Laboratory, Khabarovsk State Medical Institute MATERIALS AND METHODSThe following atriopeptides were used: 1) 23-amino acid-long atriopeptide AP-II: H2N-Ser-Ser-Cys-Phe-Gly-Gly-Arg-Ile-Asp-Pug-Ile-Gly-I I Ala-Gln-Ser-Gly-Leu-Gly-Cys-Asn-Ser-Phe-Arg-COOH. Structural characteristics and properties of this peptide are the same as those of native atriopeptides.2) 6-amino acid-long acyclic atriopeptide analog AP-H-6-OH: H2N-Arg-Ile-Asp-Arg-Ile-GIy-COOH.It was reported that similar linear analogs of atriopeptides did not bind to specific B receptors for atriopeptides [8].Experiments were performed on 4-day-old outbred albino rats. In order to diminish genetic differences control and experimental groups were formed by the method of litter split (eight pups/litter). The