Mitochondrial ultrastructure in cardiomyocytes from 3- and 24-month-old Wistar and OXYS rats was investigated using a new approach designed for morphometric analysis. The data fully confirm the electron microscopy data: the area of the inner mitochondrial membrane per unit volume of mitochondria was significantly decreased with age, as found on heart muscle section. In 3-month-old Wistar rats from the control group, this parameter was 41.3 ± 1.52 μm 2 /μm 3 , where-as in OXYS rats it was decreased to 30.57 ± 1.74 μm 2 /μm 3 . With age, an area of the inner mitochondrial membrane per unit volume of mitochondria declined in both rat strains: Wistar — from 41.3 ± 1.52 to 21.47 ± 1.22 μm 2 /μm 3 , OXYS — from 30.57 ± 1.74 to 16.3 ± 0.89 μm 2 /μm 3 . A new method that we designed and used for morphometric analysis notably simplifies the process of morphometric measurements and opens up good opportunities for its further optimization using image recognition technology.