Centrifugal serotonergic fibers innervating sensory brain areas are seen ubiquitously across systems and species but their function remains unclear. Here we examined the functional role of serotonergic innervation onto electrosensory neurons in weakly electric fish by eliciting endogenous release through electrical stimulation as well as exogenous focal application of serotonin in the vicinity of the cell being recorded from. Both approaches showed that the function of serotonergic input onto electrosensory pyramidal neurons is to render them more excitable by reducing the spike afterhyperpolarization amplitude and thereby promoting burst firing. Further, serotonergic input selectively improved neuronal responses to stimuli that occur during interactions between same-sex conspecifics but not to stimuli associated with either prey or that occur during interactions between opposite-sex conspecifics. Finally, we tested whether serotonin-mediated enhanced pyramidal neuron responses to stimuli associated with same-sex conspecifics actually increase perception by the animal. Our behavioral experiments show that exogenous injection and endogenous release of serotonin both increase the magnitude of behavioral responses to stimuli associated with same-sex conspecifics as well as simultaneously decrease aggressive behaviors. Thus, our data indicate that the serotonergic system inhibits aggressive behavior toward samesex conspecifics, while at the same time increasing perception of stimuli associated with these individuals. This function is likely to be conserved across systems and species.neuromodulation | neuroethology | excitability | neural coding A nimals must efficiently process natural sensory stimuli to successfully interact with their environment. It has become clear in recent years that sensory processing is not a passive process but instead actively depends on behavioral context (1). Adaptive control of sensory processing is in part achieved through neuromodulators such as serotonin (2). However, the functional role of serotonergic fibers emanating from the raphe nuclei innervating sensory brain areas remains largely unclear (3). This is in part because these fibers make diverse patterns of connectivity (4, 5), thereby causing a wide range of effects, such as response attenuation and gating (6, 7), as well as response enhancement (7). Thus, it is generally agreed that the function of serotonergic input onto sensory neurons is to enhance their responses to given stimulus features while attenuating responses to other features. As such, studies performed in model organisms well characterized anatomically, behaviorally, and physiologically are likely to speed progress toward a general understanding of how serotonin alters neuronal responses to natural stimuli as well as consequences on perception and behavior.The weakly electric fish Apteronotus leptorhynchus generates a quasisinusoidal electric field through the electric organ discharge (EOD) (8). Electroreceptive neurons scattered on the skin surface monitor pertu...