The changes in muscle mechanical properties caused by the myopathy of chronic uremia was examined in the soleus and the diaphragm muscles of rats in which chronic uremia was produced by subtotal nephrectomy. Using an in vitro muscle preparation in two groups of rats (moderately and severely uremic), we determined that uremia had a significant detrimental effect on both muscles with respect to the force-frequency relationships. In the diaphragm it decreased by 15% in the moderate group and by 43% in the severe group. In the soleus it decreased by 20% in both groups. Twitch characteristics behaved differently in the soleus and the diaphragm muscles in that 1/2 RT and TPT increased significantly (p < 0.05) in the soleus (severely uremic group) but not in the diaphragm. Fatigability was increased in both muscles in the moderately uremic rats and in the diaphragm in the severely uremic rats; however, the fatigability of the soleus in the severely uremic group was not different from that in the control group. Our findings suggest that the myopathic changes occurring in chronic uremia affects the function of the soleus and the diaphragm in different ways. Other findings in the severely uremic group indicate that additional factors such as marked electrolyte imbalances may also affect the excitation-contraction coupling in different ways.