In experiments on preparations of isolated spinal cord of the tadpoles, intracellular studies have been made on the synaptic potentials evoked in the lumbar motoneurones during total activation of the fibers within the 9th dorsal root. It was shown that primary afferents form monosynaptic contacts with motoneurones at stages XIV-XXV. During larval development, the number of motor cells in which monosynaptic EPSPs are recorded increases, whereas the number of motoneurones with only polysynaptic reactions decreases. From the moment of formation of monosynaptic contacts, transmission in direct sensory-motor synapses is realised by a dual (electrical-chemical) mode. The data obtained are discussed in relation to the problem of evolution of synaptic transmission between heterotypic neurones in vertebrates.