This study was performed to examine whether endothelium-derived relaxing factor (EDRF) influences venous tone and reactivity in vivo. The inferior vena cava and abdominal aorta were studied simultaneously under continuous haemodynamic monitoring in anaesthetised rabbits. In addition, a 20-MHz intravascular ultrasound catheter was placed in the vena cava for on-line two-dimensional imaging of vessel cross-sectional area and calculation of wall stress (T(ension) = P(mean) * r(adius)/2). This approach enabled simultaneous visualisation of both venous (CA(ven)) and aortic (CA(art)) cross-sectional area with continuous recording of vessel dimensions. Measurements were made before and after administration of NG-nitro-l-arginine methyl ester (L-NAME; 10 mg.kg i.v.), a specific inhibitor of EDRF biosynthesis. After L-NAME there was a significant increase in central venous pressure and a decrease in CA(ven). On the arterial side, L-NAME caused a significant increase in mean pressure and CA(art), resulting in a significantly augmented arterial wall stress. The venodilatation elicited by increasing doses of glyceryltrinitrate was markedly enhanced after L-NAME. Norepinephrine caused a parallel shift of the dose-response curve for CA(ven) in the presence of a lower baseline value. These results suggest that EDRF contributes substantially to the control of large capacitance veins in vivo and that L-NAME increases venous reactivity to both norepinephrine and glyceryltrinitrate.