1. Isometric responses to direct stimulation of inferior rectus (IR), extensor digitorum longus (EDL) and soleus (SOL) muscles and strips of diaphragm (DIA) muscle of the mouse were obtained. 2. The IR, EDL and DIA were qualitatively 'fast-twitch', while the SOL was 'slow-twitch'. The isometric twitch contraction time was for IR, 3.7 ms; EDL, 6.9 ms; DIA, 13.2 ms; SOL, 20.9 ms (at 35 degrees C). Twitch: tetanus ratio for IR was unusually small (0.12) whereas that of DIA was unusually large (0.31). 3. The force-velocity properties of the muscles were determined and the sarcomere speed of shortening calculated. The intrinsic speeds of shortening of EDL and IR were not significantly different from each other, and were 49.5 and 51.6 micrometers . s-1 respectively; the value for SOL was 23.1 micrometers . s-1 and the DIA was intermediate at 39.7 micrometers . s-1. 4. The time course of the isometric twitch of the IR was about half that of the EDL whereas the force-velocity properties of these two muscles were almost identical. This is very similar to the situation in the rat. 5. Some of the dynamic properties of the DIA were clearly intermediate between the typical 'fast-twitch' EDL and IR and the typical 'slow-twitch' SOL. This does not agree with the available histochemical evidence which suggests that the mouse diaphragm is a 'fast-twitch red' muscle.