Vomiting (emesis) is initiated by a nucleus of cells located in the medulla called the vomiting center. This center coordinates a complex series of events involving pharyngeal. gastrointestinal. and abdominal wall contractions that lead to expulsion of gastric contents. Catecholamine inhibition does not induce vomiting. Chemotherapy does not induce vomiting from autonomic instability. Chemotherapy. especially oral agents. may have an irritating effect on the gastric mucosa. which could result in afferent messages to the solitary tract nucleus. but these pathways do not project to the vomiting center.