1. The response of the isolated gastric central pattern generator (CPG) to bath application of proctolin is characterized and compared with the previously analyzed behavioral response. 2. Proctolin had an excitatory effect on the ongoing spontaneous rhythm of "combined" preparations, in which the stomatogastric ganglion (STG) is connected to the esophageal and commissural ganglia by the stomatogastric nerve (STN). The effect started between 20 s and 5 min and was characterized by strongly increased burst durations as well as increased spike rates in all units except the two lateral posterior gastric (LPG) motoneurons. The effect was strongest in the dorsal gastric (DG) and lateral gastric (LG) motoneurons and was accompanied by a phase change of the DG burst. DG continued spiking throughout large parts of the burst of LG and of the gastric mill (GM) motoneurons, which are antagonists of DG. 3. The threshold concentration was approximately 10(-10) M, and the effects were dose dependent and reversible. 4. LG and DG were identified as target cells for the action of proctolin. In LG regenerative plateau properties were induced, as revealed by its long-lasting plateau potentials, sensitivity for triggering inputs, and the occurrence of oscillatory prepotentials. An induction of endogenous bursting in DG was concluded from preparations, in which DG was cycling alone or bursting with a much shorter period duration than other gastric neurons. Hyperpolarization of DG, which normally has no or weak driving power within the gastric network, demonstrated that under the influence of proctolin, firing of DG can accelerate the gastric rhythm from a 27- to a 9-s period duration. 5. Proctolin does not only have a modulatory influence on an ongoing rhythm, but it also can trigger gastric activity. This function was first concluded from proctolin-treated STGs, which, unlike normal preparations, continue bursting if inputs via the STN are blocked. Finally, triggering was demonstrated directly, since isolated STGs that were not oscillating started a gastric rhythm after 20-30 min of perfusion with proctolin. 6. The proctolin-induced changes of the CPG activity in isolated preparations are in agreement with the effect on gastric mill chewing in the intact animal, in which, depending on the dose, different modes of chewing could be elicited.