Blood pressure was recorded during fatiguing and nonfatiguing isometric contractions of a slow-twitch muscle (the soleus) and a mixed muscle (the medial gastrocnemius) of the cat. Four tensions were examined in each muscle; 10, 25, 40, and 70% of the muscle's initial strength (tetanic tension of the unfatigued muscle). All experiments were also repeated at two muscle temperatures, 28 and 38 degrees C. For the soleus muscle, there was no change in the blood pressure during isometric contractions. For the medial gastrocnemius muscle, both the systolic and diastolic blood pressure increased markedly when either all or just the fast-twitch motor units were stimulated; however, when only the slow-twitch motor units were stimulated, a lower pressor response was observed. Venous blood samples were drawn before, during, and after fatiguing and nonfatiguing contractions of both muscles to determine the K+ concentration in the venous blood. The mean increase in the K+ concentration during contractions was 0.6 meq/l for the slow-twitch motor units of the soleus and 5.1 meq/l for the motor units in the medial gastrocnemius.