Serotonin (5-HT) is a conspicuous neuromodulator of sensory-motor networks that affects a variety of neurons at different levels of the network hierarchy. Due to its many possible targets, it has been difficult to obtain a comprehensive picture of how 5-HT achieves its final modulatory output on any given network. Our hypothesis is that the profile of 5-HT actions is dictated by its pattern of release from endogenous sites. We tested this hypothesis in the leech nervous system by means of a selective serotonin reuptake blocker (SSRI), fluoxetine. Fluoxetine evoked barrages of synaptic potentials in identified sensory, motor and inter-neurons. This effect was mimicked by the tricyclic antidepressants imipramine and clomipramine, and by the SSRI citalopram, with relative efficacies that matched their known relative selectivities for the 5-HT transporter. The synaptic responses evoked by fluoxetine in different neurons were temporally correlated, suggesting that they had a common origin. The profile of the synaptic responses matched that expected from the activation of the mechanosensory pressure cells, known to act via polysynaptic pathways. The results suggest that endogenous 5-HT acted upon cord spanning interneurons. On the other hand, bath-applied 5-HT evoked a different effect than the SSRI. Taken together, the results evidenced that the pattern of action of the monoamine is dictated by the spatial distribution of the 5-HT release sites.