1. The motor program controlling the rhythmic movements of the pyloric region of the lobster stomach is generated by neurons in the stomatogastric ganglion. Neurons within this "pyloric network" were tested for their ability to generate bursting pacemaker potentials (BPPs). For these experiments, each neuron to be tested was isolated from other cells within the stomatogastric ganglion by use of the dye-sensitized photoinactivation technique. All extraganglionic inputs to the stomatogastric ganglion were blocked. 2. Only one cell in the pyloric network, the anterior burster (AB) cell, continued to generate BPPs when isolated in this manner. The AB cell is also the only interganglionic interneuron in the pyloric network. 3. Two other neurons that were previously thought to be endogenous bursters, the pyloric dilator (PD) cells, fired tonically when isolated from synaptic input. However, tonic stimulation of the stomatogastric nerve elicited BPP generation in the PD cells. These membrane-potential oscillations were generated for periods of up to a minute following cessation of the stimulus. 4. Stimulation of the stomatogastric nerve also elicited BPP generation in two other pyloric cell types: the ventricular dilator (VD) and lateral pyloric (LP) cells. 5. A reduced subset of the pyloric network was obtained by photoinactivation of several cells, including the spontaneously oscillatory AB interneuron. When inputs to this reduced network were blocked, all pattern generation ceased. A short stimulus to the stomatogastric nerve induced an episode of coordinated patterned activity from these cells. Thus, BPP production contributes to the production and stability of the pyloric motor program.