In order to study the action of serotonin (5-HT), noradrenaline (NA), hypertensin (HT), prostaglandins A<sub>1,</sub> B<sub>1</sub> and E<sub>2</sub> (PGA<sub>1,</sub> PGB<sub>1</sub> and PGE<sub>2</sub>) and vasopressin (VP), internal carotid arteries were isolated in situ from both cerebral and general circulation and perfused continuously with oxygenated Ringers’ bicarbonate solution. The order of potencies of the vasoactive substances when administered intra-arterially was: 5-HT > HT > PGE<sub>2</sub>- > PGB<sub>1</sub> > NA. The relative duration of the constrictor effects was: 5-HT < PGA<sub>1</sub><sup>-</sup> < HT and PGE<sub>2</sub> < PGB<sub>1</sub> and NA < VP. The relaxation index of these substances on the vascular wall was: 5-HT < PGE<sub>2</sub> < HT < PGB<sub>1</sub><sup>-</sup> < NA < PGA<sup>1</sup> < VP. Some of these substances, specifically PGB<sub>1</sub> PGE<sub>2</sub> and VP, frequently caused a residual constriction of the smooth muscle following their dilator effect. The role of these vasoactive substances in the development of vasospasm is discussed.