Motor pattern generating networks that produce segmentally distributed motor outflow are often portrayed as a series of coupled segmental oscillators that produce a regular progression (constant phase differences) in their rhythmic activity. The leech heartbeat central pattern generator is paced by a core timing network, which consists of two coupled segmental oscillators in segmental ganglia 3 and 4. The segmental oscillators comprise paired mutually inhibitory oscillator intemeurons and the processes of intersegmental coordinating intemeurons. As a first step in understanding the coordination of segmental motor outflow by this pattern generator, we describe the functional synaptic interactions, and activity and phase relationships of the heart intemeurons of the timing network, in isolated nerve cord preparations, In the timing network, most (∼75%) of the coordinating intemeuron action potentials were generated at a primary spike initiation site located in ganglion 4 (G4). A secondary spike initiation site in ganglion 3 (G3) became active in the absence of activity at the primary site. Generally, the secondary site was characterized by a reluctance to burst and a lower spike frequency, when compared with the primary site. Oscillator intemeurons in G3 inhibited spike activity at both initiation sites, whereas oscillator intemeurons in G4 inhibited spike activity only at the primary initiation site. This asymmetry in the control of spike activity in the coordinating intemeurons may account for the observation that the phase of the coordinating intemeutons is more tightly linked to the G3 than G4 oscillator intemeurons. The cycle period of the timing network and the phase difference between the ipsilateral G3 and G4 oscillator intemeurons were regular within individual preparations, but varied among preparations. This variation in phase differences observed across preparations implies that modulated intrinsic membrane and synaptic properties, rather than the pattern of synaptic connections, are instrumental in determining phase within the timing network.