1. Hypertension was produced experimentally in three groups of rabbits by atherosclerosis, medial sclerosis and renal encapsulation.
2. The sensitivity of afferent baroreceptor fibre recordings, obtained from an isolated perfused aortic arch preparation, was reduced in all three treated groups.
3. The reduction of baroreceptor sensitivity was directly related to the increase in the lability of the blood pressure in the intact animal and to the reduction of the distensibility of the perfused region.
4. There was a closer relationship between the length of time the rabbits had been hypertensive and the reduction in the baroreceptor sensitivity, than to the level of their blood pressure.
5. The reduction of baroreflex sensitivity obtained by the infusion of phenylephrine was also directly correlated with the period of the hypertension.
6. Baroreceptor resetting occurred to a higher pressure in the renal hypertensive group.