A state of fluid flux probably resulting from ion movement across the plasma membrane occurs during early pregnancy or trophoblastic disease, manifesting as emesis or hyperemesis gravidarum or hydatidiform moles. In emesis or hyperemesis gravidarum, excessive secretion induced by a humoral agent may trigger vomiting by distending and activating the gastrointestinal (GI) tract mechanoreceptors. This agent may be human chorionic gonadotropin (hCG). High-affinity hCG binding sites similar to those in the ovary were found in the duodenum and pancreas of female rats, with dissociation constant values of 0.11 ± 0.02, 1.9 ± 0.6, and 4.7 ± 3.5 nM, respectively. The isoelectric point for duodenal and ovarian proteins was 5.5. With the use of two antisera directed against amino acid residues 24-33 and 239-249 of the lutropin receptor, positive immunohistochemical staining was seen in smooth muscle, Brunner's glands, parasympathetic ganglia, crypt cells, and blood vessels of the duodenum, in zymogen granules of acini, and in intralobular ducts and blood vessels of the pancreas. Under nonreducing conditions, 150- and 170-kDa proteins were seen, through Western blot analysis, in the pancreas, duodenum, and ovary. Administration of hCG to female rats in vivo caused a significant increase in HCO₃ and K⁺ secretion from the duodenum and pancreas. We hypothesize that during pregnancy hCG stimulates excessive secretion of electrolytes (and fluid) into the upper GI tract, which culminates in the vomiting during pregnancy.