1. The gastropyloric receptor (GPR) cells, which are described in the preceding paper, are a set of proprioceptive cells in the crabs Cancer borealis and Cancer irroratus that contain serotonin (5-hydroxytryptamine, 5-HT) and choline acetyltransferase. These cells have a variety of synaptic effects on cells in the stomatogastric ganglion (STG). We used pharmacologic methods to distinguish the effects that were due to acetylcholine (ACh) from those that could be due to serotonin. 2. The GPR cells evoke excitatory postsynaptic potentials (EPSPs) in two gastric mill motor neurons [lateral and dorsal gastric (LG and DG)] in the stomatogastric ganglion. The EPSPs exhibit nicotinic pharmacology, indicating that they may be due to the release of ACh from the GPR cells. 3. A train of GPR action potentials induces plateau potential properties in the DG motor neuron. This plateau potential induction is not blocked by nicotinic or muscarinic antagonists, suggesting it might be due to serotonin released from the GPR cells. Bath-applied serotonin induces a tonic depolarization of DG with high-intensity spiking. 4. In the accompanying paper, it is shown that DG-evoked muscle contraction leads to the excitation of GPR2 through mechanical coupling of the muscles. Because GPR2 also excites DG, a positive feedback loop exists between GPR2 and DG. This reflex loop may be involved in the control of the medial tooth of the gastric mill. 5. GPR stimulation initiates or enhances rhythmic pyloric cycling. This is due at least in part to a direct enhancement of bursting in the pyloric dilator/anterior burster (PD/AB) pacemaker cell group and can outlast the period of GPR stimulation by up to 1 min. GPR-induced PD burst enhancement continues in the presence of nicotinic and muscarinic antagonists, indicating that the effect is probably not due to the release of ACh. Bath application of serotonin mimicks the neuromodulatory effect of GPR stimulation on the PD/AB group by inducing or enhancing bursting. 6. Thus the GPR cells elicit at least three different synaptic actions in the stomatogastric ganglion: 1) classical, fast nicotinic cholinergic EPSPs that may be important for reflex functions in the gastric mill; 2) noncholinergic, cycle-by-cycle plateau potential induction that might be critical for the timing and operation of the gastric mill, and 3) prolonged, noncholinergic burst enhancement in pyloric neurons that is mimicked by serotonin, lasts many cycles, and may act to assure that the pyloric central pattern generator (CPG) is activated and cycling strongly.