1. By supplying pulses to different subdivisions of the ventral nerve roots in rotation, it was possible to obtain smooth contractions of cat soleus with low rates of stimulation.2. After contracting isometrically the muscle was subjected to constant velocity lengthening or shortening movements.3. During shortening the tension always fell below the isometric value. The fall in tension was usually greatest when low rates of stimulation were used.4. The effect of lengthening on tension depended on the rate of stimulation. At high rates of stimulation the tension during lengthening always rose above the isometric tension. At lower rates of stimulation (5-15 pulses/sec) the tension rose at the beginning of an extension, but decreased later in the movement to a level that was often less than the isometric tension corresponding to that muscle length. At these stimulus rates the tension during isometric contraction was usually higher than during a sustained movement in either direction.5. At low rates of stimulation longitudinal vibratory movements of more than 0.1 mm also reduced the tension far below the isometric value, whereas the reduction was quite slight when the rate of stimulation was high.6. The isometric tension during smooth contractions at low stimulus rates was remarkable in the following respects: it developed rather slowly, it was higher than the tension during or immediately after movements, and it was only slowly regained after movement had ceased.7. The results are discussed in relation to the sliding filament theory of muscle contraction, which, with certain assumptions, provides an explanation for many of the findings.