Endplate free areas of the emebrane of soleus and extensor digitorum longus muscles of the rat, covered fpor 3 days by a piece of nerve from the brachial region, become locally sensitive to ACh applied iontophoretically. Mean ACh sensitivity is increased at this site in the extensor muscle from 0 in areas not covered by the nerve to 6.27 mV/nanocoulombs (nC) +/- 1.28 (S.E.), in the area under the nerve and in soleus from 0.12 mV/nC +/- 0.05 (S.E.) to 24.2 mV/nC +/- 4.6 (S.E.). Increase in sensitivity is accompained by a decrease of resting membrane potential. Mean values are (unaffected areas) 75.0 mV +/- 0.55 (S.E.) to (affected area) 65.0 mV +/- 0.56 (S.E.) in soleus and (unaffected areas) 76.3 mV +/- 0.61 (S.E.) to (affected area) 67.0 mV +/- 0.55 (S.E.) in extensor digitorum longus. These changes are local in individual fibres. Changes in ACh sensitivity and resting membrane potential in the area covered by the nerve are temporary, and after 6 days no sign of increase in sensitivity and drop in membrane potential were observed. The piece of nerve loses its potential to induce AHh sensitivity when it is heated to 98 degrees C for 5 min or soaked for 2 h in distilled water.