The conditioned stimulus consisted of 2 a.c. electrical impulses delivered one second apart to the left rear foot. One second later the unconditioned stimulus, a 0.2 sec. d.c. shock, was applied to the right rear foot, producing a 4 inch flexion reflex. Each of 4 dogs were completely conditioned. Following complete transection of the spinal cord, 1000 additional conditioning trials were given to each dog in 10 separate sessions of 100 trials each. Two distinct response tendencies were obtained in the limb to be conditioned: a slight muscle-twitch or flexing jerk of the leg and, secondly, the crossed-extension reflex. Reasons are presented for regarding the muscle-twitch reaction as a part of the natural response to the conditioned stimulus alone and not as a conditioned response. The changes in the frequency of the muscle-twitch responses are interpreted as cases of reflex sensitization rather than conditioning. 23 references. (PsycINFO Database Record (c) 2006 APA, all rights reserved)