1. Four factors contribute to pattern generation in the pyloric network of the lobster stomatogastric ganglion. These are: a) endogenously oscillating neurons; b) synaptic network properties; c) nonlinear cellular properties, including the generation of plateau potentials; and d) excitatory input from the commissural ganglia. The roles and relative importance of these factors were investigated with a new technique for inactivating single specific identified neurons. 2. In stomatogastric ganglia in which the excitatory input is left intact, a) pattern generation continues when any cell or pair of cells other than the endogenous bursters are inactivated, b) pattern generation also continues when the endogenous bursters are inactivated, c) pattern generation ceases when the endogenous bursters plus one other particular cell are inactivated. This cell, although not an endogenous burster, displays a strong tendency to generate plateau potentials. 3. In stomatogastric ganglia that have been isolated from excitatory input, a) pattern generation continues when any cell or pair of cells other than the endogenous bursters are inactivated, b) pattern generation ceases when the endogenous bursters are inactivated. 4. Some of the inputs to the stomatogastric ganglion normally fire in bursts. However, their potentiation and acceleration of the output pattern are also produced by tonic stimulation of the nerve. The effect of one of those inputs is mimicked by bath application of dopamine to the stomatogastric ganglion. 5. The roles and importance of the three most important factors were qualitatively summarized in a chart specifying the activity of the network as a function of its intactness.