The relationship between the morphological development and the contents of glycogen, LDH, SDH in the beating as well as beat arrested cardiocytes cultured in vitro was studied cytochemically in neonatal rats. The results showed that. (1) After the cardiocytes were inoculated, they did not start to beat until they developed to elongated cells with some processes or stellated cells, and the contents of glycogen, LDH and SDH in cardiocytes reached to a given threshold (score) respectively. In beat arrested cells the contents of glycogen, LDH and SDH were below the beat-starting thresholds. (2) The greater the cell density of cardiocytes was, the more strongly the cell clusters beated. The cell density of cardiocytes was increased by mitosis. In 48-72h, cell divisions occurred in part of cardiocytes. Most mitotic figures were observed in 120h. When mitosis was in progress, cardiocytes containing glycogen particles became short but still with some branched processes. As the cardiocyte clusters were formed, the beat occurred strongly. The cytochemical reactions mentioned above may be used to evaluate how serious the cell is damaged in cytotoxicological and pharmalogical research.