To induce fatigue resistance in the latissimus dorsi muscle of the dog in preparation for possible myocardial assistance, eight adult male beagles underwent unilateral electrical stimulation of the thoracodorsal nerve at a frequency of 2 Hz (120 stimuli/min) and 10 Hz (600 stimuli/min) for a 6-week period. The conditioned muscles were compared with their unconditioned contralateral controls by fiber typing, pyrophosphate gel electrophoresis, isometric characteristics, and fatigue rates. At the end of the period of stimulation, the conditioned muscles had a greater percentage of slow-twitch, fatigue-resistant fibers on acid and alkaline stains (100 +/- 0.7% and 83 +/- 15.3%), respectively, than did their contralateral controls (45 +/- 7.6% and 43 +/- 7.0). Pyrophosphate gel electrophoresis revealed an increase in the slow myosin and a decrease in the fast myosin content in the conditioned muscles; the stimulated muscles also demonstrated a slower contraction time (87 +/- 20 msec vs. 57 +/- 17.9 msec), a lower initial tension (4.4 +/- 1.45 kg vs. 7.2 +/- 2.11 kg), and a slower fatigue rate during a 30-minute fatigue test than did their contralateral controls. The muscles stimulated at 2 Hz had fatigue rates similar to those stimulated at 10 Hz, but generally had less diminution in muscle fiber diameters and less interfiber connective tissue. Thus, it is possible to make canine latissimus dorsi muscles more fatigue resistant, and, theoretically, more capable of myocardial assistance by electrical stimulation of the thoracodorsal nerve at a frequency as low as 2 Hz--the natural canine heart rate.