In both animals and humans, stimuli leading to sympathetic activation are accompanied by an impairment of the baroreceptor-heart rate reflex. To determine whether sympathetic activity normally interferes with this reflex function we examined in conscious Wistar-Kyoto (WKY) rats the effect of chemical sympathectomy by 6-hydroxydopamine on the bradycardic response to baroreceptor stimulation induced by raising blood pressure via intravenous phenylephrine boluses; control rats received vehicle. Spontaneously hypertensive rats were also studied because in these animals there is both a baroreceptor reflex impairment and a sympathetic overactivity. Baroreceptor reflex sensitivity, calculated as the ratio of the peak increase in pulse interval to the peak increase in mean arterial pressure, was 75% greater in sympathectomized WKY rats than in control WKY rats (L28±0.15 versus 0.73±0.10 msec/mm Hg, mean±SEM; p<0.0l). The sympathectomy-induced increase in sensitivity was even larger in spontaneously hypertensive rats (SHR) (1.26±0.12 versus 0.44±0.06 msec/mm Hg in sympathectomized SHR versus control SHR, +186%; p<0.01) so that the impaired baroreceptor reflex sensitivity observed in control SHR as compared with control WKY rats (-40%, p< 0.01) was no longer detectable in the sympathectomized groups. To establish whether the sympathectomy-induced potentiation of the reflex was due to an increase in cardiac responsiveness to vagal stimuli, we subjected separate groups of anesthetized, vagotomized SHR and WKY rats to graded electrical stimulation of the right efferent vagus. The bradycardic effects of vagal stimulation, however, were similar in sympathectomized and control animals. It is concluded that 1) sympathetic nerve activity normally exerts an antagonistic effect on the baroreceptor-heart rate reflex, 2) this phenomenon is much more pronounced in SHR than in WKY rats and may contribute to the baroreceptor reflex impairment typical of the former animals, and 3) the sympathectomyinduced potentiation of the reflex does not depend on an increase in cardiac vagal responsiveness.