We have studied the lateral diffusion of acetylcholine (ACh) receptors in the extrajunctional region of developing myotomal muscle cell membrane of Xenopus tadpoles by a technique of local inactivation. The myotomal muscle cell surfaces of Xenopus tadpoles were exposed to external solution by gently removing the skin of the tail. The density of ACh receptors was monitored by membrane depolarizations in response to iontophoretically applied pulses of ACh. A pulse of alpha-bungarotoxin was pressure ejected onto the exposed fiber surface, resulting in a rapid local inactivation of the ACh receptors. With time, the functional ACh receptors diffused into the region of inactivation, producing a recovery of ACh response. That the observed recovery of ACh sensitivity is due to diffusion of ACh receptors from the unexposed undersurface of the fiber to the inactivated region was evidenced by the following: (1) no recovery was observed following prolonged toxin application; (2) pretreatment of the muscle cells with concanavalin A, which cross-links and immobilizes ACh receptors, prevented recovery; (3) mapping of ACh response along the muscle cell axis showed that the recovery cannot be accounted for by diffusion along the longitudinal axis of the fiber; and (4) the diffusion coefficients observed after scaling the recovery rate with fiber radius fell within a small range (1.5 to 4.0 X 10(-9) cm2/sec), consistent with diffusion of ACh receptors around the fiber circumference. This finding of rapid lateral diffusion within developing tadpole myotomal muscle membrane supports the notion that the localization of ACh receptors induced by innervation could be achieved by a "diffusion-trap" mechanism where the nerve contact region serves as a trap for rapidly diffusing receptors in the membrane.