It is known that the behavior of fast and slow skeletal muscles differs, but a comparison between fast and slow muscles with different stimulation patterns at various muscle lengths is lacking. The twitch, tetanus, and double pulse responses with a number of interval times have been investigated for a fast (EDL) and a slow (soleus) muscle of the rat at three muscle lengths (in vivo, at 37 degrees C, pentobarbital sodium anesthesia). The twitches of EDL and soleus change with muscle length, but the relative form of the ascending phase is independent of muscle length. The tetanus amplitude is independent of the muscle length over a considerable length range below the optimum twitch length. The form of the ascending phases of EDL-tetanus and double pulse responses (with certain interval times between the stimuli) varies with muscle length in contrast with the comparable soleus force patterns. The maximum slope in the descending phase of all contraction patterns decreases with increasing muscle length above the optimum twitch length.