The aim of this study was to investigate the effects of melatonin on the intestinal motility of hyperthyroidism rats. Therefore, we determined in vivo and in vitro effects of melatonin on duodenal tissue in experimental hyperthyroid rats. 34 Wistar-Albino male rats were fed with physiological conditions, and then euthanized by cervical dislocation. The experimental animals, Group 1: Control group (n=5), Group 1B: Melatonin group in vitro (n=5), Group 1C: Melatonin group in vivo (n=6), Group 2: 2A: Hypertension group (n=6), 2B: Group 2: Hyperthyroidi sm in vitro melatonin group (n=6), 2C: Hyperthyroidism in vivo melatonin group (n=6). Acetylcholine (ACh, 10-7, 10-6, 10-5, 10-4, 10-3, 10-2 M), potassium chloride (KCl, 20, 40, 60, 80, 100 mM) at the end of the incubation period different doses were given to the bathing environment. In in vitro melatonin groups, the determined submaximal doses (ACh 10-4 M, KCl 60 mM) and melatonin at different doses (Mel 10-10, 10-9, 10-8, 10-7, 10-6, 10-5 and 10-4 M) were applied. It was determined that the contraction responses of the isolated duodenal tissues induced by KCl and ACh increased significantly (p