In order to find how the aerobic capacity and the ultrastructure of the skeletal and heart muscle cells of endurance training rats change as a result of adding two dietary supplements with different compositions to their diet (our own dietary formula specially designed for endurance sports and Aufbau Konzentrat, a product of Pfrimmer, Germany)--we carried out an experiment with 16 Wistar rats divided in two groups. The animals from group A received a food mixture containing 76% standard rat chow and 24% of the endurance training supplement, and those from group B--a food mixture of 76% standard rat chow and 24% Aufbau Konzentrat. Both supplements contained a high-quality protein, but those designed for endurance training was of higher fat and lower carbohydrate content. The animals were subjected to training on a treadmill at submaximal loading for 8 weeks. Initially and at the end of the experiment (week 9) VO2max-test was performed and VO2max and RQ variables of the treated animals were evaluated. At the end of the experiment the maximum oxygen intake was greater in group A animals than that in group B rats--85.06 +/- 8.81 and 75.77 +/- 4.91 ml O2.kg-1.min-1, respectively. After eight weeks a material from m. gastrocnemius and from the left cardiac ventricle of all animals was obtained and treated for transimmersion electron microscopy. Morphological data for more intensive oxidative processes in the gastrocnemius muscle and myocardium of group A animals were evident. The greater number of mitochondria and the higher cristae density observed in the gastrocnemius muscle cells indicated a better aerobic adaptation induced by the training and the intake of the specially designed supplement for endurance sports. Within an 8-week period of submaximal training using the specially adapted food formula resulted in a better structural basis than Aufbau Konzentrat produced in the skeletal and heart muscle cells for improvement of the aerobic capacity.