A sexually dimorphic circuit switch in higher olfactory centres In Drosophila the male sex pheromone cis-vaccenyl acetate (cVA) elicits different behaviours in males and females: while males are repelled by cVA, it stimulates female receptivity. However, olfactory receptor neurons and their second-order partners, the olfactory projection neurons, do not show significantly different physiological responses to cVA. Using in vivo whole-cell electrophysiology I have identified two distinct clusters of third-order olfactory neurons that are pheromone-responsive only in males or females, respectively. These clusters are present in both sexes and share a common input pathway, but sex-specific wiring reroutes pheromone information. To my knowledge this is the first functional characterisation of a bidirectional circuit switch in any organism, and provides a simple mechanism for sex-specific activation of conserved motor programmes. Investigating the genetic logic of this switch, I found that the action of the fruitless transcription factor is both necessary and sufficient for the sex-appropriate wiring of these third-order olfactory neurons. Critically, expression of the male form of fruitless in females is also sufficient to masculinise the pheromone responses of both classes of neurons. Somewhat surprisingly, even selective genetic masculinisation of third-order neurons is sufficient to masculinise their morphology and pheromone responses; thus a complex neural circuit can be functionally rewired by the cell-autonomous action of a switch gene.