Measurements of the time sequence of postactivation antidromic motor nerve responses in cats after administration of physostigmine or hydroxyanilinium ions were carried out. These measurements revealed that the earliest centripetal spike in a repetitive train originates at the distal end of the motor axon with a delay of several msec, after transmission of the orthodromic volley to the muscle. This time lag for setting up the first retrograde spike can be shortened by application of two orthodromic volleys in close succession. It is inferred that the generating event for retrograde activity outlasts the occurrence of transmission, and that changes of the interval required for initiating the first retrograde spike reflect excitability changes of the most distal node of Ranvier.