Nerve terminal-smooth muscle relationships were studied in pulmonary arteries of the cat using 5-hydroxydopamine to help differentiate adrenergic and nonadrenergic terminals. There was a periarterial plexus of nerves in the walls of pulmonary arteries that extended into the lung to innervate even small arteries having a single layer of smooth muscle cells. Adrenergic nerves surrounded all arteries and extended into the tunica media of the large arteries. There were also apparent cholinergic nerves around the pulmonary arteries, although this was confirmed by electron microscopy for medium- and small-sized arteries only. The relationships of nerve terminals to smooth muscle cells in pulmonary arteries suggest that release of norepinephrine by adrenergic terminals can produce both decreased compliance and increased resistance in the pulmonary vascular bed, and that acetylcholine released by cholinergic terminals may act directly on vascular smooth muscle or on adrenergic terminals to modulate release of norepinephrine. These results suggest that both sympathetic and parasympathetic nerves may have a regulatory role in the pulmonary circulation.