The contractile characteristics of skeletal muscle were evaluated after phosphocreatine depletion was accomplished by feeding weanling female rats a ground Chow diet containing 1% of beta-guanidino-propionic acid for 10 weeks. Except for a slight decrease in initial strength and a 30% decrease in wet weight, phosphocreatine depleted plantaris muscles exhibited no abnormalities. By contrast, phosphocreatine depleted soleus muscles exhibited several significant abnormalities. In an isometric twitch, the rise time was prolonged from 57 +/- 6 ms (mean +/- S.D.) for 6 control rats to 87 +/- 6 for 6 phosphocreatine depleted rats, the amplitude was decreased from 264 +/- 11 g per g wet weight of muscle to 163 +/- 9, and the half-relaxation time was prolonged from 61 +/- 6 ms to 85 +/- 8. In addition the maximum velocity of shortening was reduced from 3 to 2 muscle lengths per second, the maximum isometric tension was reached at a lower frequency of synchronous stimulation (30 versus 50 HZ), the initial strength was reduced from 1,065 +/- 76 g per g wet weight of muscle to 752 +/- 69, and the isometric endurance was increased from 340 +/- 16 s to 973 +/- 95. The molecular basis for these effects of phosphocreatine depletion on the contractile characteristics of soleus muscles remains to be elucidated.