Because of the difficulty in isolating precapillary or postcapillary segments of the pulmonary micro circulation, there is little information on their reactivity even though they are important in normal lung function and play a crucial role in the pathogenesis of pulmonary hypertension. In order to study the reactivity of micro-vessels in lung we have developed a method in which thin slices of tissue are incubated in vitro with a vasoactive drug and are then fixed, embedded, and sectioned at a thickness of 1μm. Vessel profiles are analyzed on a computer graphics tablet, and their degree of constriction is determined morphometrically. In this system, muscular arteries less than 200 μm in diameter in adult rat lung explants are moderately constricted, but they constrict further when incubated with l-norepinephrine (10⁻⁴, 10⁻⁵ M), an effect maintained for 30 min. Nonmuscular vessels (arteries and veins) less than 100 μm in diameter also constrict in response to similar doses of norepinephrine. Although the effector cells remain to be identified, it is likely that pericytes and precursor muscle cells in the walls of these vessels are at least partly responsible.