The protocols described in this unit are designed to assess the effects of substances on intestinal transit and gastric emptying and to evaluate their ulcerogenic potential on the stomach and duodenum of the rat. Examples of results obtained with atropine or morphine (intestinal transit), loperamide (gastric emptying), or indomethacin (ulcerogenic activity) used as reference substances are provided for illustrative purposes. Atropine and morphine clearly reduce intestinal transit. Atropine, morphine, and loperamide clearly reduce gastric emptying. Indomethacin shows a marked ulcerogenic potential.