The aim of the present study was to investigate the endothelial function in human mesenteric arteries with specific reference to defining the role of endothelium-derived nitric oxide (EDNO) and the endothelium-derived hyperpolarizing factor (EDHF). Isolated segments of small human mesenteric arteries (225-450 μm inner diameter) were mounted in organ baths for recording isometric tension. In arteries precontracted with U46619 (thromboxane A2 analogue, 10-7 M), endothelium-dependent relaxations were induced in a concentration-dependent manner by substance P and histamine. In normal Krebs solution the relaxations to substance P (10-9 M) and histamine (10-7 M) were not significantly affected by preincubation with Nω-nitro-L-arginine (L-NNA, 10-4 M) or indomethacin (10-5 M). When the preparations were exposed to a solution containing 60 mM KCl, stable contractions were induced, but relaxations could still be induced by substance P and histamine. When the arteries were further preincubated with L-NNA, the relaxations were almost abolished. A combination of apamin (3 x 10-7 M) and charybdotoxin (10-9 M) almost abolished relaxations in normal Krebs solution. It is concluded that isolated human mesenteric arteries respond to substance P and histamine with relaxations that are endothelium-dependent. Synthesis of both EDNO and EDHF seem important for these relaxations, whereas prostaglandins seem to be of minor importance.