Abstract: Motilin, so called because of the initial observation that it stimalated motor activity in the stomach, is a polypeptide produced by the previonslly well recognised enterochromaffin cells. A rapit RIA for motilin has been developed with the use of antisera to conjagated motilin raid in rabbits. No significant cross-reactivity could be detected with SS, BB, PP, ACTH, ANP. ¹²⁵I-motilin as the tracer was prepared using Lodogen method. The standard range was 100 to 3200pg/ml. Fasting plasma motilin con. with 43 normal subjects was 326±58 pg/ml （normal range: 210 —442 pg/ml）. Its sera levels in 11 health men were 482.5±88.5pg/ml. After the meal of a one hour, plasma motilin values were 340±92.5pg/ml.