Individual cardiomyocytes were isolated from hearts (n = 3, 7, 3 for 3-week, 8-week, and 1-year, respectively) from DBA/2, and (n = 4, 8, 4) from C57BL/6 mice. Approximately 200 cardiomyocytes were counted from each heart. A) Mononucleated cardiomyocytes are significantly larger in both strains with aging. B) Binucleated cardiomyocytes are significantly larger with aging for both strains, and DBA/2 cardiomyocytes are also significantly larger than C57BL/6 by 1 year of age. C) Tri-nucleated cardiomyocytes increase in size between 3 weeks and 8 weeks of age for both strains, and DBA/2 mice have significantly larger cardiomyocytes compared to C57BL/6 by 8 weeks as well as at 1 year. D) Quadri-nucleated cardiomyocytes are significantly larger from 3 weeks in the C57BL/6 mice at 8wks and at 1 year of age for both strains. #: indicates significant from 3 weeks of age, *: indicates significant between strains; p < 0.05, 2-way ANOVA.