Gaining insight into the mechanism of generation of goal-directed actions is understanding neural function. In this study we examined the role of the action potential (AP) in a single molluscan neuron (responsible for a defensive response) in an instrumental behavior. The intracellular electrical activity of two neurons was recorded simultaneously. One neuron was trained and the other served as a control neuron. When the trained neuron produced an AP in response to a conditioned stimulus (CS), the mollusc did not receive a painful stimulus. Delivery of the painful stimulus did not depend on the response of the control neuron. The number of AP's in a trained neuron, the AP latency and the threshold revealed a bell-shaped dependence on learning, whereas the response of the control neuron to a CS decreased during learning. It is apparently feasible to elaborate this type of instrumental reflex, so that the discharge of a single neuron may serve as an instrumental action for the entire animal. The membrane potential in a trained neuron varies significantly during instrumental learning, but the change do not correspond to the dynamics of the instrumental reaction in the response to a CS. The control neuron exhibited weak but significant hyperpolarization during learning. The onset of the EPSP is determined by the timing of AP generation in presynaptic neurons. However, it changed in the trained neuron the elaboration of a instrumental reflex. The alterations in the latency of EPSP's during learning were significant, but were not consistent with the time history of the conditioned response. Therefore, although the learning procedure was directed to only one neuron, the presynaptic neurons and neurons at the same neuronal level (command-like neurons of the same behavior) participated in the learning. The sign of the participation was not necessarily the same as that in the trained neuron.