The area postrema (AP) has been implicated as a chemoreceptor trigger zone for vomiting (emesis) for over 40 years. The AP is located on the dorsal surface of the medulla oblongata at the caudal end of the fourth ventricle. It is one of the so-called circumventricular organs that serve as an interface between the brain parenchyma and the cerebrospinal fluid (CSF)- containing ventricles. The AP lacks a specific blood-brain diffusion barrier to large polar molecules  (i.e., a “blood-brain barrier”) and is thus anatomically positioned to detect emetic toxins in the blood  as well as in the CSF.  The NTS (nucleus of the solitary tract) may serve as the beginning of a final common pathway by which different emetic inputs trigger vomiting.