By means of a chronically implanted electrolytic transducer, changes in the aortic calibre were studied in conscious rats during the onset of acute hypertension produced by subdiaphragmatic aortic constriction. The rise of pressure to its maximum (153 +/- 5 versus 101 +/- 4 mmHg during the control period) was immediate after aortic constriction and hypertension remained stable during the entire period of observation. An equivalent (50 mmHg) but transient increase in pressure produced an increase of 0.8% in mean aortic calibre, whereas after 6 h of hypertension, dilation was 1.2%, the distensibility remaining unchanged. However, in both circumstances the displacement of the diastolic calibre was greater (80%) than the increase in pulsation (20%). After 48 h of hypertension, distensibility increased, allowing the aorta to achieve maximal dilation (6.8%) and maximal pulsation. Therefore the time taken (48 h) for the aorta to achieve a new state of resting diastolic equilibrium in acute hypertension coincides with the time course for complete resetting of the aortic baroreceptors, demonstrated before in the same preparation.