Numerous researchers have shown that experimentally induced hypertension opens the blood-brain barrier (BBB), whereas physiologically induced hypertension is accompanied by sympathetic activation, which exerts a protective effect on the BBB via cerebral vasoconstriction. It has not yet been established that transient reflex sympathetic hypertension can open the BBB. In this study, 14 lightly-anesthetized adult cats were subjected to electrical stimulation of a tooth and transient reflex sympathetic hypertension (duration of less than 60 s) was elicited repeatedly. Continuous flowmetry of the cerebral blood flow showed that autoregulation breakthrough occurred. Light halothane anesthesia supplemented with 30 or 60% nitrous oxide, or 1.2% halothane anesthesia occasionally suppressed the elicited pressor response and prevented such breakthrough. Leakage of Evans blue (EB), which was administered before the hypertensive insult, was confirmed in the marginal and suprasylvian gyri in 5 cats. The EB positive cats reached a significantly (p < 0.05) higher mean arterial blood pressure (198 +/- 16 mmHg) during reflex sympathetic hypertension than EB negative cats (189 +/- 19 mmHg). Breakthrough occurred 16 times in EB positive, but only 8 times in EB negative cats. In conclusion, transient reflex sympathetic hypertension can elicit cerebrovascular autoregulation breakthrough and if the breakthrough occurs repeatedly it is followed by the opening of the BBB.