Spiral strips from dog saphenous veins were contracted by 2.5 × 10<sup>–9</sup> M ergotamine, a concentration giving about 50% of maximal response. Phentolamine (3.6 × 10<sup>–6</sup> m) reduced these effects by 60% and indomethacin (2.8 × 10<sup>–7</sup> m) by 30%. Indomethacin was more effective in reducing ergotamine-induced increase of prostaglandin E-like activity in the bathing fluid than those induced by noradrenaline or potassium chloride. Arachidonic acid was more effective in increasing tension in ergotamine-stimulated than in noradrenaline- or potassium-stimulated veins. These findings suggest that the long-lasting venoconstrictor activity of ergotamine is mediated mainly by α-adrenoceptors but that enhanced formation of prostaglandin E-like substance(s) may also contribute.