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We studied the effects of sustained changes in resting mean arterial pressures (MAP) on arterial baroreceptor properties in anesthetized rabbits and on the baroreceptor-heart rate reflex in conscious animals. The rabbits had balloons implanted round their aorta and vena cava, for producing transient changes in MAP about the resting MAP. Aortic baroreceptor function curves were obtained at different resting MAP by relating balloon-induced changes in MAP to either (1) integrated aortic nerve activity or (2) unit baroreceptor activity. Any sustained change in resting MAP reset the unit function curves in the same direction within 15 minutes by altering their threshold without affecting gain. The effect was reversible and independent of starting pressure. It was the same whether resting MAP was altered by vasoactive drugs (nitroprusside, phenylephrine) or was changed by withdrawing or infusing blood in sympathetically …