Abstract The prodromal syndrome of radiation sickness is characterized by nausea and vomiting but the pathophysiology and the treatment of this entity is largely unknown. We investigated this problem by determining the effects of ionizing radiation on gastric function with and without administration of the dopamine antagonist domperidone. We measured gastric electrical control activity (waves per minute), fractional emptying rate (percent per minute), acid output (microequivalents per minute), and plasma levels of immunoreactive β-endorphin. Twelve conscious, chair-adapted rhesus monkeys were studied twice before, once immediately after, and once 2 days after a single 800-cGy (800 rads) 60Co total body irradiation. In addition to causing vomiting, total body irradiation transiently suppressed gastric electrical control activity, gastric emptying and gastric secretion, while increasing plasma levels of immunoreactive β-endorphin. Domperidone had no effect on vomiting or gastric function either before or after irradiation, but it significantly increased plasma immunoreactive β-endorphin.