Nerve conduction patterns of the ulnar, hypoglossal and medial brachial cutaneous nerves were determined and recorded electronically in vivo on anesthetized dogs. The recorded tracings obtained were generally found to be in accordance with those described in the literature. However, our tracings showed a new additional fact, i.e., that motor and sensory peripheral nerve fibers have a specific unidirectional conduction (centrifugally for motor and centripetally for sensory), as long as these nerves were maintained undisturbed in their natural connective tissue bed. Although these facts correspond to what is considered normal biological behavior of nerve fibers, when in their natural environment, as far as we know, this has not been demonstrated before in neurophysiological experiments.