Myocardial cells cultured from ventricles of 4-day embryonic chick hearts were labeled with H3-thymidine for the radioautographic study of DNA synthesis and nuclear cycling. After the myoblasts had been in culture for 48 hours and had begun to grow out and contract, the cultures were exposed to H3-thymidine, 1 μc/ml of Hank's balanced salt solution, for one hour. Pairs of cultures were fixed in methyl alcohol for analysis every two hours over a subsequent period of 60 hours. The total percentage of labeled cells, together with labeled mitotic figures which developed after labeling, determined the average values for generation time and nuclear cycle phases as follows: generation time, 45 hours; DNA synthesis, nine hours; G2 interval, 2–6 hours; G1 interval, 27–33 1/2 hours; and mitosis, one-half to one hour. Autonomous control of DNA synthesis was observed occasionally in a binucleate cell having only one labeled nucleus, although both nuclei were in a similar phase of mitosis. It is evident from these data that cultured myocardial cells synthesize DNA similarly to other cells.