During early development, rat soleus muscle fibres are innervated by several axons. Neuromuscular activity is involved in the elimination of all but one terminal, but it is not clear whether electrical or mechanical activity is important. Here, we reduced mechanical activity only, by interfering with excitation-contraction coupling. Muscles treated with dantrolene sodium at 9 days produced significantly less force at 13 days of age than normal muscles, and their sensitivity to ACh was greater than that of controls. The elimination of polyneuronal innervation occurs between days 9-12, but in muscles treated with dantrolene, the loss of synapses was slower. Thus, reducing mechanical activity by interfering with excitation-contraction coupling, (a) delays muscle development and (b) reduces the rate of elimination of polyneuronal innervation.