We recorded isometric and isotonic twitches, in situ, from the cat soleus at various muscle lengths and temperatures. At a given temperature the duration of isometric twitches increased approximately 60% for each 10% increase in muscle length, which was primarily owing to decreases in the rate of relaxation. For the relaxation of isometric twitches recorded at different muscle lengths, the equivalent activation energies determined were the same (13.2 +/- 0.3 kcal/M). The duration of isotonic twitch contractions increased only 20% for each 10% increase in muscle length. Even a small amount of shortening (3%) diminished the dependence of twitch duration on muscle length. In this case, twitch duration increased approximately 30% for every 10% increase in muscle length. Hence, even small changes in internal and/or external compliance (eg, changes in the tendon-fiber continuity) can greatly influence twitch duration. Our findings are consistent with the hypotheses that in the cat soleus, Ca2+ sequestration is primarily governed by a single energy dependent process and that the Ca2+ sensitivity of the contractile apparatus increases with increasing sarcomere length.