The less-complex central nervous system of many invertebrates make them attractive for not only the molecular analysis of the associative learning and memory, but also in determining how neural circuits are modified by learning to generate changes in behavior. The nudibranch mollusk Hermissenda crassicornis is a preparation that has contributed to an understanding of cellular and molecular mechanisms of Pavlovian conditioning. Identified neurons in the conditioned stimulus (CS) pathway have been studied in detail using biophysical, biochemical, and molecular techniques. These studies have resulted in the identification and characterization of specific membrane conductances contributing to enhanced excitability and synaptic facilitation in the CS pathway of conditioned animals. Second-messenger systems activated by the CS and US have been examined, and proteins that are regulated by one-trial and multi-trial Pavlovian conditioning have been identified in the CS pathway. The recent progress that has been made in the identification of the neural circuitry supporting the unconditioned response (UR) and conditioned response (CR) now provides for the opportunity to understand how Pavlovian conditioning is expressed in behavior.The analysis of learning in several vertebrate and invertebrate nervous systems has generated a number of candidate mechanisms of Pavlovian conditioning involving changes in both cellular excitability and synaptic strength. In general, studies of associative learning in these model systems have been dominated by the search for mechanisms of Pavlovian conditioning that provide an explanation for temporal contiguity between the conditioned stimulus (CS) and unconditioned stimulus (US). However, a comprehensive analysis of associative learning requires an understanding of all aspects of the associative process, including the generation of behavior, in addition to providing insights into mechanisms of temporal contiguity. The issue of how learning is expressed in behavior may be effectively addressed by studying conditioning from a cellular and synaptic perspective in the relatively simple nervous system of invertebrates. The analyses of learning in several model systems have used a combination of cellular and synaptic physiology in conjunction with a neural circuit analysis to examine how Pavlovian conditioning is expressed in behavior, or how learning results in the generation of a conditioned response (CR). Some invertebrate preparations are especially attractive for this type of analysis, as the neural circuitry supporting behaviors involving muscular contraction, respiration, locomotion, and feeding is known in considerable detail (for review, see Sahley and Crow 1998).One animal that has contributed to an understanding of the physiology of learning and memory at a cellular, synaptic, and systems level of analysis is the nudibranch mollusk Hermissenda crassicornis. Associative learning in Hermissenda has been extensively examined using a Pavlovian conditioning procedure. The Hermissenda central...