The effect of several pharmacologic agents on isolated human and canine pulmonary vein strips suspended from a force transducer was tested. Serotonin, norepinephrine and epinephrine caused contraction in most segments tested. Digitoxin was found to induce contraction of the pulmonary vein segments after a variable latent period. Tolazoline and aminophylline caused relaxation in a large number of the veins tested, although relaxation of human vein strips in response to tolazoline was less consistent. This study adds to the knowledge of the reactivity of human pulmonary veins.