Single-channel properties of nicotinic acetylcholine receptors in Xenopus embryonic muscles were investigated by the patch clamp technique. Dissociated muscle cells were prepared from embryos in early stages of development (stages 18-19) before innervation takes place, and were cultured without neurons for 4-6 days. Despite the absence of innervation in their history, the cells displayed two classes of acetylcholine receptors, one characterized by a small channel conductance (32 pS), and the other by a large conductance (48 pS) (13 degrees C; agonist, suberyldicholine). The small conductance events had longer mean open times than the large conductance events. Both types of channels had reversal potentials near -15 mV. Hyperpolarization prolonged the open times of both channels; an e-fold change was produced by a 70-80 mV polarization. In order to characterize channel properties of newly inserted receptors, existing receptors were inactivated by alpha-bungarotoxin, and recordings were made over the next 4-8 h. These new receptors already exhibited the two classes of characteristics, which were similar to those of old acetylcholine receptors. These results suggest that innervation is not a prerequisite for expression of the two classes of acetylcholine receptors, and indicate that receptors become functionally mature soon after insertion into the plasma membrane. These results of 'metabolically' new receptors are in sharp contrast with the data of Leonard et al. (Soc. Neurosci. Abstr., 9 (1983) 1180), who reported that 'ontogenetically' new acetylcholine receptors had much longer open times than old receptors.