1. This is a report of experiments carried out on the cat and on man, which demonstrate that conditioning of a muscle by contraction and movement can lead to changes in amplitude of stretch reflexes elicited in that muscle. 2. In triceps surae of the cat, the reflex response to a brief stretch was recorded after conditioning with a whole-muscle contraction followed by a pause at a length either 5 mm longer or shorter than the length at which the reflex was elicited. Following conditioning at the long length the reflex response was less than half as large as that following conditioning at the short length. 3. The changes in reflex amplitude could be correlated with an altered stretch responsiveness of muscle spindles in the soleus muscle. When the muscle had been held long during conditioning, a subsequent brief stretch applied at an intermediate length elicited fewer impulses in primary endings of spindles than after conditioning at a short length. 4. The same kind of experiment was then carried out on adult human subjects. When a tendon tap was applied to the Achilles tendon after a voluntary contraction and relaxation of triceps surae with the muscle at a long length, (foot dorsiflexed) the reflex was frequently less than half the size it had been after a contraction at a short length (foot plantarflexed). It was concluded that the same kind of spindle aftereffects as observed for cat soleus spindles were responsible for the changes in reflex amplitude. 5. It was found both in the cat and in human subjects that the changes in reflex amplitude after conditioning became progressively less as the test length was made longer. 6. The explanation put forward to account for these observations is that stable cross-bridges form between actin and myosin filaments of passive intrafusal (and extrafusal) fibers. When the muscle is shortened several seconds after a contraction at a long length, the intrafusal fibers, stiffened by the presence of cross-bridges, fall slack. Slack does not develop after a contraction at a short muscle length, as the fiber is stretched to the test length. Since any slack must first be taken up by the test stretch, there is a smaller afferent response and consequently a smaller reflex contraction in response to a tendon tap after conditioning at a long length.(ABSTRACT TRUNCATED AT 400 WORDS)