The neurological basis of behavior is poorly understood on a genetic and cellular level, such understanding is crucial to the dissection of mental diseases. By elucidating the role of the male-specific products of the fruitless gene (fruM) in establishing an innate, stereotypic, and complex behavior, an understanding will be gained regarding how a behavioral circuit is determined and regulated in a genetically tractable organism. The aims of the project are threefold. (I) Identify fruM-expressing neuronal populations that are dedicated to regulating the activation of the male courtship routine. II) Characterize the role these cells by courtship-related sensory assays to define functional roles for fruM-expressing neurons in early courtship. Ill) Image the neuronal projection patterns and manipulating synaptic transmission in target fruM-expressing neurons. Addressing these aims will provide insight into the functions of the fruM-expressing neurons in regulating a defined behavioral circuit. Sex-specific differences in neuronal morphology and number will provide clues into how the sex-specific circuitry is established in the male brain.