1. Prolonged inactivity of soleus (slow twitch) and medial head of flexor digitorum longus (fast twitch: previously commonly known as flexor hallucis longus and hereafter referred to as FDL) muscles of the cat was produced by sodium pentobarbitone anaesthesia or spinal isolation. Isometric contraction characteristics were examined after 4-22 days and 8-49 days respectively.2. Sleep of up to 3 weeks' duration was associated with progressive changes in the weight, maximum tetanic tension, speed of contraction and absolute refractory period of fast twitch muscle. The slow muscle showed corresponding changes which were not so pronounced. Cord isolation resulted in similar changes which tended to be reversed after longer periods of disuse.3. Fast muscle frequently exhibited an after-contraction with corresponding electromyographic activity following two weeks or more of disuse.4. The time to peak of FDL was prolonged, becoming approximately one and a half times normal. In contrast to other reports of disuse, that of soleus was normal or slightly prolonged.5. The ratio of twitch to tetanic tension of FDL increased to twice the normal value or greater and remained high. The soleus muscle showed a smaller increase with evidence of a return towards normal after longer periods of inactivity.6. Absolute refractory period of FDL was considerably prolonged (more than 50%) following barbiturate sleep and this also occurred, but to a lesser degree, with soleus after 4-7 weeks of cord isolation.7. The observation that inactivity produced slowing rather than an increased speed of switch contraction indicates that aggregate activity is not the controlling factor. Changes seen in the contraction characteristics of FDL are noted to show a general similarity to those occurring following denervation.