Synapses with the complex nerve endings consisting of several terminal branchings of a single axon divided by myelin segments were studied at the cutaneous-pectoris muscles of a frog by histological and electrophysiological methods. It has been revealed that the number of the synapses with complex nerve endings rises during the postnatal development. The relation between the nerve ending complexity, total terminal length and size of the muscle fibres is described. Some of branches in complex nerve endings arise from the Ranvier nodes. They are characterized by a low level of quantal secretion and a peculiar fall of the sodium current along nerve terminals. Mechanisms that induce growth of nerve endings, branching and peculiarities of the transmitter release are discussed. It is supposed that the processes of growth and myelinization of nerve endings take place simultaneously.