Object. Cerebral vasospasm following subarachnoid hemorrhage (SAH) leads to reduced blood flow in the brain. Inspired by organ culture—induced changes in the receptor phenotype of cerebral arteries, the authors investigated possible changes in the 5-hydroxytryptamine (HT) receptor phenotype after experimental SAH. Methods. Experimental SAH was induced in rats by using an autologous prechiasmatic injection of arterial blood. Two days later, the middle cerebral artery (MCA), posterior communicating artery (PCoA), and basilar artery (BA) were harvested and examined functionally with the aid of a sensitive in vitro pharmacological method and molecularly by performing quantitative real-time reverse transcription—polymerase chain reaction (PCR). In the MCA and BA the 5-HT_1B receptor was upregulated, as determined through both functional and molecular analysis. In response to selective 5-HT₁ receptor agonists both the negative logarithm of the 50% effective concentration was increased (one log unit in the MCA and one half unit in the BA), as was the agonist's potency (increased by 50% in the MCA and doubled in the BA). In addition, the authors found an approximately fourfold increase in the number of copies of messenger RNA coding for the 5-HT_1B receptor as determined by quantitative real-time PCR. In the PCoA no upregulation of the 5-HT_1B receptor was observed. Conclusions. Changes in the receptor phenotype in favor of contractile receptors may well represent the end stage in a sequence of events leading from SAH to the actual development of cerebral vasospasm. Insight into the mechanism of upregulation may provide new targets for developing specific treatment against cerebral vasospasm.