Aging is associated with suppressed baroreflex function. Renal sympathetic nerve activity was recorded from young (1 year) and old beagles (11 years) during a step rise in isolated carotid sinus pressure. An abrupt increase in pressure resulted in a significant and similar inhibition of efferent nerve activity in both groups, but the inhibition was not sustained in the old as compared with the young animals. The escape from sympathetic inhibition in the old could not be explained by a decline of input from sensory baroreceptor neurons. Thus the defect in the aged animals is caused by a rapid adaptation of central baroreflex neurons to the baroreceptor input instead of a lack of responsiveness of these neurons, suggesting a functional rather than a structural impairment.