1. The role of vasoconstrictor 5-HT1-like receptors in the control of vascular reactivity in vivo has been relatively little studied, particularly with regards to venous function. Using an anaesthetized dog model, we have investigated the haemodynamic profile of the selective 5-HT1-like agonist, sumatriptan, focussing on the reactivity of the saphenous venous bed. The key feature of our experimental model was the implantation of ultrasonic crystals on the adventitial surface of the lateral saphenous vein to provide direct and continuous measurement of drug-induced changes in vein diameter. Saphenous vein pressure was measured simultaneously via a proximal branch. 2. Sumatriptan 1-30 micrograms kg-1, i.v., produced pronounced dose-related reductions in saphenous vein diameter which reached congruent to 40% at the highest dose tested. Sumatriptan also produced modest increases in mean blood pressure, total peripheral resistance and left ventricular end diastolic pressure but had little or no effect on cardiac output, heart rate, cardiac contractility or saphenous venous pressure. Sumatriptan-induced reductions in saphenous vein diameter were strongly antagonized by the 5-HT1-receptor antagonist, methiothepin (0.3 mg kg-1, i.v.) but were unaffected by the 5-HT2 antagonist, ketanserin (0.3 mg kg-1, i.v.). 3. Hence, 5-HT1-like receptor stimulation in vivo can result in a powerful local venoconstrictor effect.