Different age groups of male Wistar rates were submitted to intense strength training, swim training or no training. It was found that the tetanic of the m. soleus decreased with age. This was counteracted by strength training, whereas swim training had no effect on tetanic tension. Force at ultimate failure and yield point in the Achilles tendon decreased with age, but were not influenced by strength training. Swim training, on the other hand, appeared to compensate for the ageing process in the tendon. We conclude that tendon strength decreases with age and that tendon strength does not reflect muscle strength. The absolute tensile strength of tendons appears to be influenced by physical activity of endurance-type exercise. However, it is not known whether the tissue responds to the number of mechanical muscle contractions or physiological factors connected with endurance training. Although it is questionable to extrapolate from the rat to humans, it is speculated that intensive muscle strength training should be accompanied by endurance exercise in order to prevent the muscles from damaging connective tissue structures, such as the tendons and ligaments.