Pharmacological effects of dobutamine (DOB), a positive inotropic drug, on smooth muscles were compared with those of catecholamines such as norepinephrine (NE), epinephrine (E), isoproterenol (ISO) and dopamine (DA). DOB acted as a partial agonist and simultaneously as an antagonist on alpha-adrenergic receptors in the thoracic artery. Large doses of DOB induced uterine contraction in rabbits, but antagonised the contraction of the vas deferens of guinea pigs, as was induced by electrical stimulation. However, DOB showed a potent activity, similar to that seen with NE or E as an alpha-adrenergic agonist, on the portal vein. Doses of DOB required to produce a relaxation in the trachea of guinea pigs both in vitro and in vivo were 1/250 and 1/600 of ISO, respectively, and were considerably less regarding inhibition of uterine motility in rats. Inhibiting activity of DOB on the transport rate of intestinal contents in mice and gastro-intestinal motility of rabbits was much less than the activity seen with ISO, E and NE. Spasmolytic activity of DOB was also less than that of ISO, NE and E, but was more than that of DA. It is concluded that DOB is a more selective beta 1-agonist than ISO because of its weaker activity on alpha- and beta 2-receptors. The alpha-agonist activity of DOB in the portal vein was however, potent.