It is well established that neural networks respond to a wide variety of modulatory substances by which they can become reconfigured, yet few studies have examined the effects of neurotransmitter mixtures on such networks. In a previous study of the medicinal leech using triple intracellular recordings, we found that stimulation of identified mechanosensory neurons activated both the serotonergic cell 21 (a swimgating neuron) and the dorsal lateral octopamine (DLO) cell. Because these findings suggested that serotonin (5-HT) and octopamine (OA) may be released together, we investigated the effects of 5-HT and OA mixtures on isolated nerve cords of Hirudo medicinalis (which contained both head and tail brains). Fifty micromolar OA, 50 μM 5-HT, or a mixture of 50 μM OA and 50 μM 5-HT was bath applied to the nerve cord under constant perfusion conditions. Additional experiments were performed with combinations of either 25 or 100 μM OA and 5-HT. Neural activity was examined specifically in the segmentally repeated dorsal posterior (DP) nerve because it has been shown to contain identified swim motor units. Nonadditive effects of amine combinations were most apparent in their ability to decrease overall activity in the DP nerve and to alter patterned motor activity in the form of fictive swimming. Whereas swim burst activity has been previously shown to increase in nerve cords bathed in either 5-HT or OA solutions alone, we demonstrated that a mixture of the two amines resulted in a robust decrease in the number of swim bursts expressed and an inhibition of swim activity in preparations already swimming. Most compelling was the observation that when the amine mixture was replaced with normal saline, swim burst activity increased dramatically. We discuss that the effects of amine mixtures may be due to their interaction with descending interneurons known to trigger and inhibit swimming as the mixture-induced effects were not observed in nerve cords lacking the head and tail brains. Because the net effect of the two amines was not simply additive (i.e., 5-HT or OA is known to activate swimming, yet the mix inhibits swimming), this result reveals yet another layer of complexity inherent in "simpler" invertebrate nervous systems.