Electrophysiological study of dually innervated end-plate sites was carried out in Xenopus laevis pectoral muscle fibres. End-plate potentials (e.p.p.s) and miniature end-plate potentials (m.e.p.p.s) have been recorded in Mg-blocked preparations. 2. The mean quantal content (m) of each e.p.p. at dually innervated end-plates was significantly smaller than the corresponding values obtained at singly innervated ones. At a given doubly innervated end-plate site the values of m tended to be inversely related, so that the compound value of m (obtained by adding them) was in the same range as that found in singly innervated junctions. These findings were taken to suggest the existence of an upper limit in the average amount of transmitter released at a synaptic site. 3. It was found that neither intermittent presynaptic conduction block, nor particular muscle fibre properties could account for the low values of m in dual end plates. The small size of the nerve terminals appears to be the most likely explanation. 4. The sensitivity to acetylcholine and muscle fibre electrical properties were investigated; no differences were found between fibres with sub- or suprathreshold e.p.p.s. 5. The nature of the factors responsible for this presumed small size of the nerve endings (competition between nerve endings for a limited synaptic space on the muscle membrane or reciprocal interaction between closely located terminals) as well as the possible origins of polyinnervation are discussed.
