SUMMARY1The arterial baroreceptors are reset to operate at higher pressure levels in hypertension. Rapid (acute) resetting occurs within the first few minutes after elevation of arterial pressure, but is only partial because the increased threshold for baroreceptor activation represents only 25‐50% of the arterial pressure increase.2Complete resetting occurs when the increase in pressure threshold equals the increase in arterial pressure; in the rat this is present after 48 h hypertension.3The aortic calibre was studied in freely moving rats; the time taken for the diastolic calibre to reach maximum dilation correlated with the time taken for complete resetting of the threshold of the aortic baroreceptors. During transient pressure increases the displacement of the diastolic calibre was much greater than the increase in pulsation, indicating that, under physiological conditions, sustained distension of the diastolic calibre is an important factor in aortic baroreceptor distortion.4The relative change of the diastolic calibre, in relation to control calibre, remains relatively constant during transient pressure changes in aortae of increased calibre produced by chronic hypertension or growth.5It is concluded that complete resetting of the baroreceptors in hypertension occurs when the increased stress on the arterial wall is matched by a proportional increase in diastolic calibr