In senile (28- to 36-month-old) rats prolongation of latency period, contraction and relaxation time in fast muscle is confirmed; however, there is a shortening of these time parameters in the slow soleus muscle. Ca2+-activated myosin ATPase activity decreases in fast but increases in the slow senile soleus muscle. A trend of levelling out of the differences between fast and slow muscles is thus observed in respect to speed of contraction and myosin ATPase activity.