1. It has previously been shown that the antimigraine drug sumatriptan constricts porcine carotid arteriovenous anastomoses via 5-HT1-like receptors, identical to 5-H1B/1D receptors. The recent availability of silent antagonists selective for the 5-HT1B (SB224289) and 5-HT1D (BRL15572) receptor led us to further analyse the nature of receptors involved. 2. In pentobarbitone-anaesthetized, bilaterally vagosympathectomized pigs, sumatriptan (30, 100 and 300 microg kg(-1), i.v.) dose-dependently decreased carotid arteriovenous anastomotic conductance by up to 70+/-5%. 3. The dose-related decreases in carotid arteriovenous anastomotic conductance by sumatriptan (30, 100 and 300 microg kg(-1), i.v.) remained unchanged in animals treated (i.v.) with 1 mg kg(-1) of BRL15572 (maximum decrease: 72+/-3%), but were significantly attenuated by 1 mg kg(-1) (maximum decrease: 30+/-11%) and abolished by 3 mg kg(-1) (maximum decrease: 3+/-7%) of SB224289. The highest dose of SB224289 did not attenuate the hypertension, tachycardia or increases in carotid blood flow induced by bolus injections of noradrenaline (0.1-3 microg kg(-1), i.v.). 4. The results indicate that sumatriptan constricts porcine carotid arteriovenous anastomoses primarily via 5-HT1B, but not via 5-HT1D receptors.