When a hind-limb muscle nerve or a lumbar dorsal root of rats was stimulated, a fast and well-synchronized wave could be lead off from the appropriate lumbar ventral root and was named ‘the fast synchronous wave’. While this fast synchronous wave resembled in shape and appearance to the cat monosynaptic reflex, it differed in some features from the latter, i.e., rather long latency, absence of post-tetanic potentiation and not being most excitable of all the reflex components. To testify whether the fast synchronous wave is monosynaptic or not, and if monosynaptic, to what causes its characteristics should be attributed, electrophysiological technique and Nauta method were employed. It was concluded that the fast synchronous wave was monosynaptic and its specific features were due to the poor functional connection between primary afferents and motoneurons.