These studies address the apparent dissociation between the amounts of norepinephrine (NE) released from small and large pulmonary arteries of dogs by a standard electrical stimulus and the contractile tensions that developed in these tissues. Segments of vessels were studied in organ baths or in a superfusion apparatus during electrical stimulation. Endogenous NE was quantitated using liquid chromatography with electrochemical detection. ‘Release’ of NE was studied under conditions in which uptakes of NE from the synaptic cleft were impaired, and overflow of NE was studied when these uptakes were operative. ‘Release’ of NE was predictably greater than overflow in all arteries. In the large pulmonary artery, greater contractile tensions were measured when greater amounts of NE were present in superfusate. In the small pulmonary artery, contractile tensions did not directly correlate with the amounts of NE measured in the superfusate. These studies indicate that NE dynamics are different at neuroeffector junctions in large and small pulmonary arteries. Several explanations to account for these differences are discussed.