1. The interactions among the four pairs of interneurons (HN(1)-HN(4)) of the heartbeat timing oscillator are confined to the third and fourth ganglia (G3 and G4). In isolation, G3 and G4 each produces a rhythm essentially the same as that shown when the two ganglia are linked together. 2. The local circuits in both ganglia have the same general form. In both the oscillation centers on a bilateral pair of HN cells that are linked by reciprocal inhibition (the HN(3) pair in G3 and the HN(4) pair in G4). In addition, there is reciprocal inhibition between an HN(3) or HN(4) cell and the intersegmental processes of the ipsilateral HN(1) and HN(2) cells. 3. These connections account for the phase relationships in an isolated G3 or G4, since cells linked by reciprocal inhibition produce bursts in alternation. 4. In isolated ganglia, reciprocal inhibition not only coordinates the activity of the HN cells but also appears to help generate their bursts. 5. Yet reciprocal inhibition alone cannot account for the activity of the network. An endogenous property of the HN(3) and HN(4) cells appears to create the instability necessary for oscillation.