Evoked and spontaneous endplate potentials (e.p.ps) were recorded with intracellular electrodes from fibres of the rat diaphragm and the frog sartorius muscle. The amplitudes of the evoked e.p.ps were reduced to about one-half their control values in the presence of 0.1 mm and 0.2 mm of adenosine triphosphate (ATP). The amplitude of the spontaneous endplate potentials (miniature e.p.ps) was unaffected by ATP but their frequency was reduced.