Resistance to blood flow in a given vessel is proportional to the 4th power of the radius. The principal site of increased pulmonary vascular resistance is the precapillary arteriole. Yet, most studies of pulmonary vascular responses have utilized large conductance vessels. We hypothesized that vascular responses of PRV (<300 μm diameter) would differ from those of conductance-sized (>500 μm diameter) pulmonary arteries (PA) and veins(PV). PRV from newborn piglets were isolated, cannulated at both ends, and pressurized to 15 mm Hg. The vessel image was projected on a monitor for continuous measurement of the lumen diameter with a video dimension analysis system. Newborn piglet PA and PV rings were suspended in tissue baths for isometric force measurements. In vessels precontracted with the thromboxane analogue, U46619 (0.1 μM), BK (1 μM) caused more complete relaxation in PRV (94.7 ± 5.2%) than in PA (63 ± 16%) or PV (25 ± 9%). In PA and PV, BK-mediated relaxation was completely abolished by nitro-L-arginine (L-NA; 0.1 mM), an inhibitor of nitric oxide synthase. This inhibition was reversed by excess L-arginine, but not D-arginine, and was inhibited by hemoglobin. Neither glibenclamide (0.1 μM), an inhibitor of ATP-sensitive K+ channels, nor indomethacin (0.01 mM), a cyclooxygenase inhibitor, had any effect on BK-induced relaxation in PA or PV. In constrast, addition of L-NA, indomethacin or glibenclamide alone to PRV caused partial inhibition of BK-induced relaxation. Compared with 94.7% relaxation in the absence of inhibitors, BK caused 54.3 ± 4.4, 81.4 ± 8.1 and 65.2± 11.5% relaxation in the presence of L-NA, indomethacin and glibenclamide, respectively. We conclude that the signaling mechanisms mediating BK-induced relaxation are different in pulmonary resistance vessels than in large conductance vessels. These findings highlight the segmental differences in pulmonary vascular responses and the importance of including pulmonary resistance vessels in investigations of signaling mechanisms mediating the unique responses of the pulmonary circulation. (Supported by March of Dimes #6-FY96-0703)