Tranylcypromine (10 −4 and 10 −5M), but not iproniazid, pheniprazine or pyrogallol (10 −4 and 10 −5 M), increased the tension of rabbit aortic and esophageal strip and decreased the tension of guinea pig taenia coli. This effect was prevented by phenoxybenzamine and phentolamine (10 −5 M) but not by propranolol or MJ1998 (both 10 −5 M). Only pyrogallol (10 −6 M) potentiated the norepinephrine response of the aortic strip. An alpha receptor stimulant action for tranylcypromine, and the relative unimportance of MAO in smooth muscle, are suggested by these results.