Stimulus-response curves of aortic baroreceptors constructed by alternately increasing and decreasing pressure from a normal baseline or set-point differ from curves constructed by varying pressure in one direction only from an abnormally high or low pressure. In anesthetized dogs we recorded impulses from aortic baroreceptors with myelinated fibers, using a pressurized reservoir to control mean aortic blood pressure (MABP). After setting MABP to a baseline of 100 mm Hg (normal MABP in unanesthetized dogs), we constructed baroreceptor response curves by alternately decreasing MABP from 100 to 30 mm Hg, and increasing it from 100 to 180 mm Hg, in each case returning MABP to the baseline to obtain hysteresis loops. All baroreceptors were active at 100 mm Hg, their discharge averaging 15-16 impulses/sec. At all pressures above threshold, baroreceptors fired more when pressure was increasing than when pressure was decreasing. This hysteresis caused the steepest part of the response curve constructed in this manner to span the baseline value, demonstrating that, contrary to previous views, aortic baroreceptors signal decreases in pressure below the normal level, as well as increases above it. We also constructed response curves after holding MABP at a "hypertensive" baseline of 125 mm Hg for 20 minutes. "Hypertensive" curves demonstrated reversible resetting, shifting significantly to the right of "normotensive" curves so that baroreceptor threshold increased on average by 7 mm Hg (P less than 0.01). Both hysteresis and short-term resetting probably result from the viscoelastic behavior of wall elements with which baroreceptors are coupled.