Neostigmine increases the size of subunits composing the quantum of transmitter release at mouse neuromuscular junction.

Miniature end-plate potentials (m.e.p.p.s) were recorded from mouse diaphragm junctions. Noise-to-signal ratios were less than 1.3%. 1-4 X 10(3) m.e.p.p.s were recorded before and after the addition of an anticholinesterase agent. M.e.p.p. amplitude distributions showed two classes of m.e.p.p.s. The mode of the bell m.e.p.p. class was ten-twelve times that of the skew m.e.p.p. class. The amplitude distributions of the bell m.e.p.p. class showed integral peaks in the central region. The anticholinesterase agent increased the interval of the integral peaks but not the number of peaks. Experimental conditions that are necessary to demonstrate integral and stationary peaks on m.e.p.p. amplitude histograms are discussed. Data support the hypothesis that the quantum of transmitter release is composed of subunits.