The distribution of acetylcholine-sensitive membrane areas in slow muscle fibres of pyriformis muscles of Rana temporaria was examined by iontophoretic application of acetylcholine from high resistance pipettes. ACh-sensitivity varied considerably along individual slow fibres and from fibre to fibre. In some fibres the sensitivity was restricted to segments of less than 100 microns, in others it was continuous over several millimeter. Segments of variable length, but up to several millimeter, were completely insensitive to acetylcholine. Highly sensitive spots (greater than 1,000 mV/nC) were found occasionally, their diameter being of the order of 10-20 microns only. The occurrence at rather regular intervals of ACh-sensitive areas was a rare observation; no evidence was found for a generalized ACh-sensitivity. There were marked differences in the lengths of ACh-sensitive segments between surface fibres and fibres located in deeper layers of the muscles. It is concluded that the ACh-sensitive membrane areas correspond to individual nerve muscle contacts of the small motor system whose spatial distribution is extremely variable. In superficial slow fibres the synaptic contacts seem to be located predominantly on the internal circumference of the fibres.