1. The 26 identified neurons of Aplysia buccal ganglia include 4 interneurons and their follower cells. Each interneuron makes cholinergic synaptic connections on eight identified ipsilateral follower neurons. Each interneuronal action potential also produces a zero-latency, Mg-intensitive electrotonic coupling potential in one cholinergic and electrotonic input from the interneurons. Electrotonic connections are bidirectional and nonrectifying. 2. Ipsilateral pairs of interneurons receive extensive common synaptic input from several unidentified neurons: each interneuron also receives some input not received by the other. These pairs are linked by bidirectional nonrectifying electronic coupling which is insensitive to high Mg. As a consequence of this organization, ipsilateral interneuron pairs can fire a) independently, or b) synchronously, or c) one active interneuron can depolarize the other. 3. Each follower receiving synaptic input from one ipsilateral interneuron also receives similar input from the other interneuron. Common follower cells thus receive a) asynchronous PSPs, or b) large summated PSPs, or c) an increased number of PSPs from each interneuron. The latter two modes constitute feed-forward summation of interneuronal action. 4. Interneuronal output is confined to ipsilateral neurons. Symmetric pairs of interneurons are coordinated by common inputs and are not directly interconnected by either chemical or electrotonic synapses. Synchrony of firing of symmetric pairs is, therefore, looser than that of ipsilateral pairs.