During neurophysiologic studies in the rat [Crl:COBS CD(SD)BR], chloral hydrate was given intraperitoneally to produce anesthesia for brain electrode implantation. The incidental occurrence of adynamic ileus in six of these rats, 5-16 days after surgery, prompted further investigation. Pathological evaluations and transmission experiments using ingesta and viscera from affected rats failed to reveal an infectious agent. Subsequently chloral hydrate-induced adynamic ileus leading to morbidity or death was experimentally produced in 14 of 27 rats, 3--36 days after intraperitoneal administration of the anesthetic at a dose of 400 mg/kg body weight and concentrations of 125--275 mg/ml. The experimentally produced condition was characterized by lethargy, anorexia, abdominal distension, ruffled hair coat, inactivity, drowsy expression, constipation, and death. Gross pathologic findings included gaseous distension and atony of the cecum and segments of the small intestine. Small intestine contents varied from liquid to solid. The colon and rectum contained only a few dry hard fecal pellets. A few animals showed excess abdominal fluid. Microscopic findings were limited to focal chronic serositis and serosal fibrosis affecting the visceral peritoneum. On the basis of the experimental studies, it was concluded that the condition described was not an infectious disease but was an abnormal physiologic condition produced by the irritating or pharmacologic action of chloral hydrate given at high concentrations in the abdominal cavity.