A study was made of synaptic currents in voltageclamped muscle fibers of the frog. Fast, submaxillaris, and slow muscle fibers are innervated by nerve fibers of different conduction velocities. To avoid spatial complications, transmitter release by nerve impulses was restricted to the site of recording and reduced to single quanta (unitary endplate currents: uepc). Following nerve stimulation, the time course of transmitter release was longer and more variable in slow and submaxillaris muscle fibers than in the fast fibers. The time constant of decay of uepc in submaxillaris and slow fibers was, respectively, about 1.8 and 2.9 times slower than the decay of uepc in fast fibers. This is due mainly to differences in the lifetime of the channels opened by acetylcholine. The neuromuscular junctions in submaxillaris muscle fibers are boutonlike or longer branched contacts; and the unitary currents in the bouton junctions have a slower time course. It is concluded that the synaptic membrane in the different types of muscle fibers has synaptic acetylcholine-operated channels that have different kinetic properties, and that these properties are determined by the type of axon that innervates the muscle fiber.