Strips of isolated rat diaphragm muscle were attached to a servomotor-transducer apparatus, and the muscle length was cycled in a sinusoidal fashion about the length at which maximum isometric twitch force was developed, Lo. The amplitude of the length displacement (excursion amplitude) and rate of cycling were varied between 3 and 13% Lo and 1-4 Hz respectively. The muscle was tetanically stimulated (100 Hz, supramaximal voltage, stimulus duration (duty cycle) 20% of the length cycle period) during the shortening stage of the imposed length cycle at the phase that yielded maximum net positive work. The force and displacement of the muscle were recorded. Work per cycle was calculated from the area of the loop formed by plotting force against length for one full stretch-shorten cycle. Work per cycle decreased, but power increased, as cycle frequency was increased from 1 to 4 Hz. Maximum work done per cycle was about 12.8 J/kg at a cycle frequency of 1 Hz. Maximum mean power developed was about 27 W/kg and occurred at a cycle frequency of 4 Hz. Work and power were maximum at an excursion amplitude of 13% of Lo (i.e., Lo +/- 6.5%). Measured work and power output are considerably less than values estimated from length-tension and force-velocity curves.