8-Hydroxy-2-(di-N-propylamino)tetralin (8-OH-DPAT), a reported serotonin receptor agonist, was found to produce drops both in mean arterial blood pressure (MAP) and heart rate in unanesthetized spontaneously hypertensive rats. The hypotensive episodes were elicited whether the compound was given intraperitoneally (minimum hypotensive dose = 0.02 mg/kg), orally (7.5 mg/kg or intracerebroventricularly (0.014 mg/kg). No bradycardia, however, was elicited following intracerebral infusions of 8-OH-DPAT. The bradycardia and hypotension were moderate in duration (1-4 hr). The serotonin receptor blocking agents cyproheptadine (5 mg/kg i.p.) and methergoline (1.0 mg/kg i.p.) failed to reduce either of the cardiovascular actions of 8-OH-DPAT. Methiothepin (0.5 mg/kg), another serotonin receptor blocker which produces falls in MAP itself, failed to attenuate 8-OH-DPAT-induced hypotension, but did block 8-OH-DPAT-induced bradycardia. 8-OH-DPAT exerts potent hypotensive and bradycardic effects in the unanesthetized spontaneously hypertensive rat. Whether this effect is the result of serotonin receptor activation is not yet clear.