Cardiac ventricular myocytes (cvm) from old mice were classified in subpopulations of different hypertrophy and stress-tolerance to test whether individual cvm age to a different extent. In mitochondria (mt) of cvm from old and young mice, ROS were generated by photoactivation of tetramethylrhodamine methylester (TMR) with confocal linescans (lsc). Stress-tolerance was characterized by 1) half decay time (HDT) of TMR fluorescence during lsc (ROS depolarizing the mt membrane potential) and 2) possible time-dependent increments in fM, the fraction of polarized mt (ROS damaging mt). Cvm volumes were measured by 3-dimensional confocal microscopy. In comparison to young cvm, old cvm had volumes that were larger (79±20 pl versus 55±13 pl) and more heterogeneous (4 versus 2 Gaussian distributions). HDT grouped around 4 s (type2) and 8 s (type1 cvm). Type2 contributed <5% in young cvm, however, 34% in old cvm. In old type2 but not in young and old type1 cvm, fM decreased with time after lsc. DFF images suggested that diffusion of ROS damaged mt of the neighbourhood. Conclusion: In old hearts, a substantial subpopulation of cvm is strongly hypertrophied and stress-intolerant. Reduced stress-tolerance is thought to be caused by age-reduced cellular ROS defence capacity.