regional contractile responses in pulmonary artery to alpha and betaadrenoceptor agonists contractile sensitivity and reactivity to alpha and betaadrenoceptor stimulation was studied in incubated rabbit pulmonary artery cylindrical segments of differing diameters distinct differences were noted between the responses of extra and intrapulmonary pulmonary arteries to norepinephrine and isoproterenol the sensitivity to norepinephrine was largest in the intrapulmonary pulmonary arteries arterial reactivity to norepinephrine was greatest in the larger of the intrapulmonary vessel segments diminishing considerably as the vessels became smaller cocaine did not cause substantial alterations in the response of any of the arterial segments to the alphaagonist phentolamine however exerted its influence primarily in the smaller arterial segments vascular sensitivity to isoproterenol was least in the intrapulmonary pulmonary arteries these smaller vessel segments however were more reactive to isoproterenol than were the extrapulmonary pulmonary arterial segments propranolol at a concentration of 108 m was an effective antagonist of the betaagonist at a concentration of 107 m however this antagonist was related to isoproterenolinduced arterial contraction apparently by stimulation of alphareceptor sites the results of this study demonstrated a regional heterogeneity in the contractile response of the pulmonary artery to alpha and betastimulation the extrapulmonary arterial segments were found to be more sensitive to betastimulation than were the smaller intrapulmonary segments the intrapulmonary arterial segments on the other hand were found to be more sensitive to alphastimulation than were the extrapulmonary segments