Vasodilator prostaglandins are released in vitro from endothelium during adrenergic stimulation. We hypothesized that indomethacin would block this production in vivo and increase venoconstriction to ₁- and ₂-stimulation but not to the nonadrenergic agonist PGF₂. Hand vein distension was measured in 24 normal subjects (23.0 ± 0.5 yr) during local infusions of phenylephrine (8-12,000 ng/min), clonidine (3-7,000 ng/min), or PGF₂ (1-2,048 ng/min) plus indomethacin (3 µg/min) versus saline on two separate days. Dose-dependent venoconstriction to phenylephrine occurred in all subjects, with a parallel shift to the left with indomethacin (P = 0.003) and a decrease in the phenylephrine 50% effective dose (1,009 vs. 241 ng/min, geometric means, P = 0.012). Venoconstriction to clonidine was more variable, with most subjects eliciting a biphasic response (initial venoconstriction followed by attenuation). With indomethacin, the dose-response curve was displaced up and to the left (P = 0.005), and peak venoconstriction was increased (51.1 ± 6.8 vs. 27.2 ± 5.3% of control, P = 0.018) without a biphasic response. In all subjects, PGF₂ elicited dose-dependent venoconstriction that was not altered by indomethacin. Thus venous ₁- and ₂-stimulation results in release of vasodilator prostaglandins that antagonize the venoconstrictor response. This modulates the sympathetic response of venous smooth muscle and may be important in diseases with endothelial dysfunction.