The effects of 1 and 2 wk of hindlimb suspension (HS) on rat skeletal muscle function were determined and the results compared with those obtained previously with hindlimb immobilization (HI). Both models of disuse (HS and HI) primarily affected slow-twitch muscle. Each decreased the isometric twitch duration in the slow-twitch soleus; however, the HS-mediated effect was entirely a result of a shortened contraction time (CT), whereas HI reduced one-half relaxation time ( 1/2 RT) as well as CT. Soleus muscle mass and peak tetanic tension (P0) declined with disuse. The HS effect on muscle mass and P0 was variable, however, for all experiments HS produced atrophy equal to or greater than HI. A major difference existed in the effects of HS and HI on the maximal speed of soleus muscle shortening (V(max)). One and 2 wk of HS produced increases in V(max) to 4.45 ± 0.34 and 6.83 ± 0.74 fiber length/s, respectively, compared with control velocities of 3.05 ± 0.08. By contrast over a similar time period, HI had no significant effect on soleus V(max). The increase in V(max) at 14 days of HS was associated with, and perhaps caused by, the increased expression of a second faster migrating isozyme of myosin. The new native isozyme comigrated with fast myosin, but its light chain subunits contained only LC(1s) and LC(2s). The mechanism responsible for the increase is unknown. One plausible explanation is that the apparent HS-mediated modification in muscle fiber type is dependent on the elimination of loadbearing or isometric contractions, a condition that does not exist during HI.