YM-31636 free base is an orally active, potent and selective 5-HT 3 receptor agonist with a K i value of 0.2 nM. YM-31636 free base induces contraction of isolated guinea pig distal colon. YM-31636 free base induces tachycardia with the relative intrinsic activity of approximately 0.23 in isolated guinea pig right atrium. YM-31636 free base is promising for research of constipation.