The general goal of this proposal is to study the role of neuromodulators in the generation and modification of behavior. Simple behaviors are generated within the central nervous system by limited circuits of neurons called Central Pattern Generators (CPGs). CPGs can produce stereotyped motor patterns in isolation, but they depend on extensive modulatory input to produce the normal variety of related behaviors. The cellular mechanisms involved in this modulation have not been studied. I propose that modulatory inputs change the active components of the CPG by adding and subtracting cells from the circuit, changing firing characteristics of neurons, and changing synaptic efficacy within the circuit. To test this hypothesis, I will analyze the cellular and ionic mechanisms whereby serotonin, octopamine and dopamine modulate the well-characterized 14-neuron CPG for the pyloric rhythm in the stomatogastric ganglion of the lobster. Using electrophysiological and pharmacological techniques, I will answer several questions: 1) How many of the 14 identified neurons are directly excited or inhibited by an amine? 2) Are synaptic connections in the circuit modified by amines? 3) What are the ionic mechanisms for amine excitation and inhibition of each target neuron? This work will describe how generalized CPGs are sculpted by modulatory inputs to produce specific functional circuits generating a wide spectrum of behaviors.