We performed experiments to study the effects of electrolytic lesions of periaqueductal gray matter on mean blood pressure, heart rate, and cardiac baroreflex in adult male spontaneously hypertensive rats. Cardiac baroreflex was assessed by the administration of randomly assigned doses of phenylephrine (0.3 to 5.0 micrograms/kg i.v.) or sodium nitroprusside (1.5 to 5.0 micrograms/kg i.v.) to unanesthetized rats. Bilateral lesions of the periaqueductal gray matter (0.5 mA/5 s) were then performed with rats under sodium pentobarbital anesthesia (35 mg/kg i.p.). Twenty hours after lesion, cardiac baroreflex was retested. Baroreflex data were analyzed by sigmoidal curve fitting. Lesion rats (n = 12) showed a significant decrease in both the gain (delta = -0.89 +/- 0.38 beats per minute [bpm]/mm Hg, P < .05) and curve midpoint (delta = -15 +/- 6 mm Hg, P < .05) of the cardiac baroreflex. Moreover, despite a moderate increase in heart rate (delta = 34 +/- 10 bpm, P < .01), resting mean blood pressure was significantly decreased 24 hours after the lesions (delta = -19 +/- 5 mm Hg, P < 01). No significant changes in cardiac baroreflex were observed in sham-lesion rats (n = 12). Histological examination showed circumscribed bilateral damage of dorsolateral periaqueductal gray matter. Dorsolateral periaqueductal gray matter is an area of the brain putatively related to fear and anxiety. It also projects onto premotor sympathetic neurons in the medulla. Although electrolytic lesions damage neurons as well as fibers of passage, these data suggest that dorsolateral periaqueductal gray matter has a far greater influence on resting cardiovascular control in spontaneously hypertensive rats than was previously suspected.