In anaesthetized rats, supramaximal baroreceptor stimulation by carotid sinus inflation evoked a reflex fall in arterial pressure and an increase in vascular conductance and flow of muscles of the hindquarters. 2. Simultaneously, main arteries, primary and secondary arterioles (13-90 microns, internal diameter (i.d.)) and terminal arterioles (7-13 microns) of the spinotrapezius muscle all showed a diameter increase that reached a peak as arterial pressure neared its zenith; terminal arterioles then showed a diameter decrease to below control level. These responses were abolished by local application of phentolamine or guanethidine to the spinotrapezius, or by crushing the paravascular nerve supply. 3. It is suggested that the diameter increases were mediated by reflex inhibition of sympathetic tone, while the secondary diameter decrease in terminal arterioles was induced by a fall in local concentrations of vasodilator metabolites, caused by an increase in muscle blood flow. 4. But after sympathetic blockade 25% of all arterial vessels showed a diameter increase beginning as the fall in arterial pressure neared its zenith. These responses may be attributed to vasodilator metabolites accumulating as a consequence of a reduction in muscle blood flow secondary to the reflex reduction in perfusion pressure. 5. No venous vessels, from venules (9-18 microns) to main veins (65-130 microns) that drain the muscle, showed a diameter change in response to baroreceptor stimulation, in accord with evidence that they have no sympathetic supply. 6. These results accord with and can provide explanation for changes in blood flow, regional blood volume and capillary filtration evoked by baroreceptor stimulation in studies on whole-limb muscle. They support suggestions that active changes in capacity of venous vessels of muscle play a minor role in the baroreceptor reflex; both active and passive changes in vascular capacity may be due to large veins outside of muscle proper.
