Pharyngeal muscles exhibit two distinct types of contractions called pumps and peristalses. A peristalsis is a wave-like contraction followed by rapid relaxation that travels posteriorly through the isthmus, so that only a small portion of the isthmus lumen is open at any time. This contraction is remarkable, because it occurs within individual pm5 muscle cells that extend the entire length of the pharyngeal isthmus. Peristalses occur only after several pumps, and they are dependent on signals from the M4 motor neuron. We are examining how M4 and the isthmus muscles interact to produce these peristalses. M4 is a cholinergic neuron, and we have found that cha-1 mutants lacking acetylcholine (ACh) fail to pump or peristalse. Chemical stimulation of either muscarinic or nicotinic ACh receptors (mAChRs and nAChRs) in cha-1 mutants is sufficient to activate peristalsis. We have examined several mutants affecting AChR genes expressed in the pharyngeal muscles and have found that mutants defective in the nAChR eat-2 and the mAChR gar-3 exhibit distinct peristaltic defects. eat-2 mutants pump less frequently than wild-type animals, but most of the pumps are followed by peristalses. The eat-2 peristalses are prolonged suggesting that EAT-2 stimulates isthmus muscle relaxation. In comparison, gar-3 mutants fail to respond to the mAChR agonist arecoline indicating GAR-3 mediates peristalsis in response to exogenous arecoline. gar-3 mutation also suppresses the prolonged peristalses observed in eat-2 mutants, which suggests that these prolonged contractions are stimulated by GAR-3. To understand better how these AChRs affect peristalsis, we examined Ca2+ dynamics in the isthmus muscles using GCaMP3. Wild-type animals display a broad increase in Ca2+ in the center of the isthmus during pumps followed by wave-like Ca2+ transients in the posterior isthmus during peristalses. We found that changes in Ca2+ concentration during peristalsis in gar-3 mutants are similar to those in wild type, and this is consistent with our observation that gar-3 mutants do not have any defects in peristalsis under normal conditions. In contrast, eat-2 mutants have larger changes in Ca2+ concentration in the posterior isthmus during peristalsis, and these Ca2+ transients are prolonged in eat-2 mutants when compared to those in wild-type animals. Based on our observations, we conclude that GAR-3 is the receptor which mediates peristalsis in response to exogenous arecoline, but not essential for these contractions under normal conditions. Surprisingly, we found that EAT-2 may be involved in stimulation of isthmus muscle relaxation, rather than isthmus muscle contraction.