The contractile properties of plantaris and soleus muscles were studied in 46-day-old hamsters after 4 weeks of rear-end suspension; their controls were animals which had not been treated or else had been operated upon but not suspended. In the suspended hamsters the plantaris muscles, of which approximately 90% of fibers are normally type II, maintained the properties of fast-twitch muscles in terms of their contraction and half-relaxation times, maximum rates of rise of tetanic tension, and posttetanic potentiation of the twitch. The small reduction of mean tetanic tension, although not significant statistically, was compatible with relatively mild atrophy of the type II fibers; approximately half of the type I fibers appeared to have converted to type II. In contrast, the soleus muscles, normally slow-twitch with approximately 60% type I fibers, exhibited significant shortening of their contraction and half-relaxation times after suspension and the maximum rates of rise of tetanic tension were increased. These changes, and the greatly reduced twitch and tetanic tensions, were compatible with the finding of much greater atrophy of type I than type II fibers, together with the conversion of approximately 10% of fibers from type I to type II. Unlike other contractile variables, posttetanic depression of the soleus twitch was unaffected by suspension. Our findings have implications in relation to other models of disuse.