BACKGROUND Differences in the size of cardiac muscle cells observed in normal and hypertrophic hearts have been assessed through different methodologies. Spontaneously hypertensive rats are often used as an experimental model of essential hypertension in humans, which allows researchers to study the relation between hypertension and cardiac hypertrophy. It has been shown that ventricular hypertrophy in mammals progresses and ventricular failure develops in the end stage of hypertrophy. The aim of the present study was to analyse a number of morphometric markers and compare them between male normotensive Wistar rats and male spontaneously hypertensive rats. MATERIALS AND METHODS The total number of male Wistar rats was 15, distributed in five age groups, each containing three animals: two-weeks-old; one-month-old; three-months-old; six-months-old; twelve-months old. The male spontaneously hypertensive rats were distributed in two age groups, each containing three animals: one-month-old (young) and six-months-old (adult) RESULTS: As aging progressed, both in male normotensive Wistar rats and in male spontaneously hypertensive rats we noted a statistically significant increase in the morphometric parameters thickness of the free wall and the cross-sectional area of the cardiomyocytes and their nuclei and a decrease in the cardiomyocytic density in both ventricles. These changes were more pronounced and occurred at an earlier age in spontaneously hypertensive rats. CONCLUSIONS The present study analyses in detail the alterations in the myocardium of the left and right ventricle, initiated by age-related hypertrophy, as well as hypertrophy induced by arterial hypertension.