1. The potent, antimigraine drug ergotamine has affinity for both 5-HT1 and 5-HT2 binding sites and constricts arteriovenous anastomoses. Since 5-HT also constricts arteriovenous anastomoses (mainly via 5-HT1-like receptors), this study investigates the involvement of 5-HT receptors in the ergotamine-induced reduction of arteriovenous shunting in the carotid circulation of the cat and pig. 2. In the cat, ergotamine (3, 10 and 30 micrograms kg-1, i.v.) reduced carotid blood flow, predominantly by a reduction in arteriovenous anastomotic blood flow. Pretreatment with ketanserin (0.5 mg kg-1, i.v.) or methiothepin (1 mg kg-1, i.v.) did not antagonize the effects of ergotamine. 3. In the pig, ergotamine (2.5, 5, 10 and 20 micrograms kg-1, i.v.) also reduced carotid blood flow and arteriovenous shunting, which was not affected by pretreatment with methiothepin (1 mg kg-1, i.v.). 4. These results suggest that the reduction by ergotamine in the shunting of carotid arterial blood via cephalic arteriovenous anastomoses is not mediated by 5-HT1-like or 5-HT2 receptors.