The initiation of leech (Hirudo medicinalis) swimming in isolated segmental nerve cord preparations requires only excitation of segmental swim gating and swim oscillatory interneurons. However, several observations indicate that when the entire isolated central nervous system (head ganglion through tail ganglion) is used, neuronal inputs from the head ganglion other than excitatory inputs to the segmental swim-generating network influence whether swimming results in response to a given stimulus. In this study, experiments were performed to demonstrate that the initiation of swimming is controlled by two parallel pathways emanating from the head ganglion that have opposite effects on the segmental swim-generating network. One pathway, the swim-activating system, excites the segmental swim-generating network, while the other pathway, the swim-inactivating system, suppresses it. The balance between the effects that the swim-activating and inactivating systems have on the segmental swim-generating network determines whether swimming occurs. Moreover, we identified a pair of interneurons, cells SIN1, in the leech head ganglion whose spiking activity must be suppressed in order for swimming to be initiated since their activity is incompatible with swimming. Depolarization of cell SIN1 during swimming indirectly inhibits segmental swim-gating interneurons and terminates ongoing swimming activity. Thus, cells SIN1 are most likely part of the swim-inactivating system in the leech head ganglion.