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Growth and ploidy of rat ventricular myocytes were studied during development in situ and in grafts (1 day old rat ventricle transplanted under kidney capsule of syngenic adult animals). Both in situ and in the transplants polyploidization occurred on days 4-14 of postnatal life, and the modal group of myocytes was represented by binucleate diploid (2c x 2) cells. Minor quantities of 4c, 4c x 2, 8c and 2c x 4 myocytes were detected as well. In ventricles of 14 and 28 days old rats and in the transplants of the corresponding age the portion of polyploid myocytes was 90-96% and 32-63% respectively. The intensity of postmitotic myocyte transplants was decreased as compared with in situ development, and cells that exit proliferation cycle did not grow until day 14. The data on thymidine label dilution suggest that diploid myocytes of the transplant can divide two or three times but the majority of labeled diploids divided only once. Labeled 2c x 2 myocytes originated from the first, and less frequently, the second cell generation or resulted from initial acytokinetic mitosis. Mononucleate tetraploids 4c originated from 2c x 2 and mostly from 2c cells. Octaploids were formed after 3d or 4th labeled mitosis. The conclusion about cardiac myocyte polyploidization as an intrinsic developmental program is supported, implying the programming of onset, mode, duration and termination of polyploidization and its prolongation during early postnatal life.(ABSTRACT TRUNCATED AT 250 WORDS)