A single identified neuron, the medial type B photoreceptor, was isolated by axotomy from the nervous systems of nudibranch molluscs (Hermissenda) which had been exposed to three different training experiences. Paired animals had been trained with repeated paired presentations of light and rotation and random animals with randomized light and rotation; naive animals had no training. A two-microelectrode voltage clamp of axotomized type B somata (separated from all synaptic interactions and impulse activity) was used to measure, with a blind procedure, three distinct ionic currents at least 24 h after the training experience. An early K+ current, IA, and a Ca2+-dependent K+ current, ICa2+-K+, but not a light-induced inward Na+ current, were significantly reduced for the paired as compared to the random and naive animals. The magnitude of ICa2+-K+ reduction was related (again measured blindly) to the degree of training-induced suppression of phototaxis (a measure of the learned behavior) for the paired animals. These data are consistent with previous observations indicating that changes of intrinsic type B membrane properties are an important means for encoding the acquisition and retention of Hermissenda associative learning.