The stomatogastric ganglion of the lobster Panulirus interruptus contains about 30 neurons and controls the striated musculature of the stomach. The ganglion produces two complex rhythms, the pyloric cycle and the gastric mill cycle, when completely deafferented. This paper describes the neural circuitry underlying this activity in terms of interactions among motor neurons. The pyloric motor neurons are coordinated by electrotonic and inhibitory synaptic interactions which are driven by a group of three neurons having endogenous bursting capability. The gastric mill cycle does not appear to have any such driving source, and instead relies on the overall properties of the network to generate its burst patterns. Preliminary computer modeling indicates that alternate bursting between antagonists can occur without cyclically bursting driver cells. Computer reconstruction of Procion-filled stomatogastric neurons are used both to corroborate the results of the physiological studies and to quantify the geometry for purposes of modeling the intraneuronal flow of synaptic currents.