Selective staining of the connective tissue and image analysis showed that in the extensor digitorum longus and soleus muscles of the rat there was an increase in the thickness of the endomysium in both early growth and senility. The perimysium thickness was more or less constant throughout life except in senility when the concentration of this component also increased. The stiffness (length-passive tension) of these muscles was found to increase throughout life. Log transforms of the length-passive tension plots had particularly steep slopes in the senile extensor digitorum longus muscle. Except in the senile soleus muscle, the increase in stiffness was closely correlated with the increase in endomysium and perimysium and with total muscle collagen (as measured biochemically) with age. The relationship between the initial length and the active tension in the extensor digitorum longus muscle changed with age. The older muscles showed a greater decline in tension for each decrement of length resulting from the increased development of the connective tissue.