Two series of experiments were performed to examine the interrelationships between blood pressure, intramuscular pressure, muscle blood flow, and the endurance for isometric exercise in a fast (medial gastrocnemius) and a slow (soleus) twitch muscle of the cat. In the first series of experiments, the relationship between tension and intramuscular pressure was examined. It was found that intramuscular pressure was linearly related to tension in both muscles. However, at any proportion of the muscles maximum tension, the intramuscular pressure of the medial gastrocnemius muscle (the stronger of the muscles) was about twice that of the soleus. A second series of experiments was conducted in which blood pressure was increased above intramuscular pressure and the effect of blood pressure on isometric endurance was measured. The pressure of the perfusing blood of the cat's hind limb was adjusted to either 13.3, 26.6, or 39.9 kPa. It was found that increased perfusion of the muscle resulted in a dramatic increase in the endurance for contractions sustained at isometric tensions below 60% of the muscle's initial strength. In contrast, for contractions above this tension, the effect of increased perfusion was much less pronounced.