1. The defensive withdrawal reflex of the siphon of Aplysia is a local response (exhibited by the organ that is stimulated) mediated by the conjoint action of both the central and peripheral nervous systems. 2. Three independent methods were used to determine the contribution of the central and peripheral nervous systems to the siphon-withdrawal reflex: 1) acute reversible deganglionation, 2) chronic deganglionation, and 3) a selective reversible hyperpolarization. With each of these techniques, the central nervous system was found to contribute about 55% of the total reflex. 3. Seven motor neurons were identified and characterized with respect to their electrophysiological properties and the motor actions. Three of the central motor cells belong to the LD clusters of cells (LDS1, LDS2, LDS3) and one is an RD cell (RDS). These four cells all receive excitatory synaptic input from siphon stimulation, excitatory synaptic input from the activity of the respiratory command cells network (interneuron II). large spontaneous IPSPs, and exhibit hyperpolarizing responses (H response) to iontophoretically applied acetylcholine (ACh). These cells all participate in the siphon-withdrawal component of a centrally commanded fixed-action pattern: spontaneous pumping movements of the mantle organs driven by the respiratory command cells. They receive an EPSP burst during the activity of the respiratory command cells and are competent to mediate the siphon motion. Three central siphon motor cells belong to the LB cluster (LBS1, LBS2, LBS3). These cells also receive excitatory input following stimulation of the siphon, a spontaneously occurring IPSP, and have H response to iontophoretically applied ACh. These cells, however, receive an IPSP burst during spontaneous pumping movement and thus do not participate in the active contraction phase of this behavior. LBS1 and LDS1 were examined with respect to their transmitter biochemistry and were found to be noncholinergic. 4. The siphon-withdrawal reflex habituates with comparable kinetics to repeated tactile stimulation when it is under central and peripheral control and when it is under peripheral control only. Thus, not only do both systems act conjointly to produce the defensive withdrawal reflex, but also they have similar response properties and are well matched to mediate the two parts of this siphon behavior.