1. By subdividing ventral roots and supplying stimulating pulses to different groups of motor units in rotation, smooth contractions of soleus could be obtained with low rates of stimulation.2. Isometric tension was recorded with different rates of stimulation, and at different muscle lengths.3. Longitudinal histological sections were cut from muscles fixed at different lengths, and sarcomeres were measured. Mean sarcomere lengths in soleus could then be related to the angle at the ankle.4. At high rates of stimulation the maximum active tension was obtained at a length corresponding to an angle of about 60 degrees at the ankle, and a mean sarcomere length of about 2.8 mu. The isometric tension fell only slightly on shortening the muscle to a length equivalent to 100 degrees , and a mean sarcomere length about 2.3 mu. Further shortening caused a marked fall in tension.5. There was a reciprocal relationship between stimulus rate and muscle length; when the muscle was long low rates of stimulation gave near maximal tension, whereas at short lengths the maximum tension was reached only when the stimulus rate was very high. It is suggested that stimulating pulses activate the contractile machinery of the muscle more effectively at long than at short muscle lengths.6. When at low rates of stimulation pulses were distributed among the motor units in rotation to give a smooth contraction, the tension rose higher than during the unfused tetanus that accompanied synchronous stimulation of the same motor units at the same rate. It is suggested that in an unfused tetanus internal movement of the muscle reduces the tension below that developed in a truly isometric state.7. The rate of rise of tension in an isometric tetanus varied with both muscle length and rate of stimulation. At each stimulus rate there was a range of lengths in which the isometric tension developed slowly, this was the same length range in which, at that stimulus rate, the length tension curve was steep.