The steady-state force-velocity relation was examined at temperatures between 35 and 10 degrees C in rat fast-twitch (extensor digitorum longus, e.d.l.) and slow-twitch (soleus) muscle preparations in vitro, with direct stimulation, and employing the isotonic release technique. The curvature of the force-velocity relation increased with cooling in both muscles; the increase was more pronounced below 25 degrees C. The maximum shortening velocity of e.d.l. muscle decreased with a Q10 (temperature coefficient) of 1.8 in cooling from 35 to 25 degrees C; it decreased with a Q10 of 2.4 in cooling below 20 degrees C. The shortening velocity of slow muscle was more temperature sensitive. The Q10 values for soleus muscle were 2.0 at 35-25 degrees C and 3.5 below 20 degrees C. The maximum rate of isometric tetanic tension rise had a temperature sensitivity similar to the maximum velocity of shortening in each muscle type.