In humans, it has been reported that dynamic exercise attenuates spontaneous baroreceptor reflex sensitivity (sBRS), which is an index of the gain of the baroreceptor-cardiac reflex. We demonstrated previously that endogenously produced NO from endothelial nitric oxide synthase (eNOS) within the nucleus tractus solitarii (NTS), the central terminal site of baroreceptor afferents, depressed sBRS. In this study, we investigated whether eNOS activity within the NTS plays any role in down-modulating the sBRS during dynamic exercise. In conscious Wistar rats arterial pressure and heart rate (HR) were monitored continuously and chronically using radiotelemetry before and during wheel cage running at 6 m min-1 for 10 min. sBRS was determined by a time-series method. During dynamic exercise systolic blood pressure (SBP) and HR were significantly increased (SBP: 138 ± 2 vs. 125 ± 2 mmHg, P < 0.001; HR: 447 ± 6 vs. 362 ± 8 beats min-1, P < 0.001) while sBRS was significantly decreased (0.53 ± 0.03 vs. 1.08 ± 0.08 ms mmHg-1, P < 0.001). In sino-aortic denervated rats the change in SBP in response to dynamic exercise was significantly larger than that in baroreceptor-intact rats (denervated: 21.6 ± 2.5 mmHg; intact: 12.0 ± 2.8 mmHg, P < 0.05). In contrast, denervation made no difference to the change in HR. Although disabling eNOS activity in the NTS by adenoviral-directed expression of a dominant negative mutant form of eNOS increased resting sBRS (1.48 ± 0.20 vs. 1.09 ± 0.15 ms mmHg-1, P < 0.05), the absolute level reached during dynamic exercise was identical to control. These results demonstrate that during dynamic exercise (i) the sBRS decreases around the operating point of the baroreceptor-cardiac reflex function curve in normotensive rats, (ii) the baroreceptor reflex operates to limit the rise in arterial pressure, and (iii) the attenuation of sBRS is not mediated by changes in eNOS activity within the NTS. Experimental Physiology (2003) 88.4, 517-526.