1. The frequency response of isometric soleus muscle was determined efficiently by analysis of the unfused tension generated during short periods of random stimulation of the divided ventral roots, in anaesthetized cats.2. Despite the complexities of skeletal muscle, the frequency response of soleus, at moderate lengths and stimulation rates in the physiological range, is closely approximated by the frequency response function for a simple, linear, second-order system near critical damping.3. The soleus muscle shows a uniformly high sensitivity to fluctuations in nerve activity over a range of frequencies similar to the range of frequencies of muscular activity observable during behaviour. The nerve-muscle preparation appears to be well suited for smooth and steady motor activity, since it is much less responsive to the higher frequency components contained in individual action potentials or generated in tremor.4. The second-order parameters: low-frequency gain, natural frequency and damping ratio provide useful descriptions of the changes in the muscle's response caused by variations of muscle length, nerve stimulation rate or number of active motor units.5. A reduction in tension incurred, for example, during fatigue can be compensated to some extent either by a lengthening of the loaded muscle or through an increase in neural spike repetition rate. However, both mechanisms produce concomitant increases in the ;sluggishness' (increased damping and lower natural frequency) of the preparation. This sluggishness may arise out of limitations imposed by the mechanism for the re-uptake of calcium into the sarcotubular system.6. One naturally occurring method of increasing tension, by recruitment of more active motoneurones, seems to be desirable because tension can be augmented in this way without an increase in sluggishness. This is presumably because recruitment increases the muscle's response without affecting excitation-contraction coupling in fibres already active.