The effect of nerve activity on the sensitivity of the neuromuscular junction to acetylcholine was studied. After 1 h of nerve stimulation the flexor hallucis longus muscles of cats become more sensitive to the blocking effect of suxamethonium than before stimulation. Similar results were found using rat soleus muscles. The increased sensitivity to suxamethonium was still observed in muscles from rats which had been curarized during the period of stimulation. Intracellular recordings from end plates of rat soleus muscles revealed an initial transient increase in miniature end-plate potential (MEPP) frequency after stimulation and an increase in the amplitude of MEPPs which persisted for as long as 4 h. Moreover the depolarising responses to acetylcholine and suxamethonium added to the bath were always greater in stimulated muscles. It was therefore concluded that activity of the motor nerve increases the sensitivity of the end plates to transmitter.