1. The behaviour of forty-four single carotid baroreceptor fibres from nine dogs with normal blood pressure was contrasted with that of forty-seven single fibres from nine dogs with renal (nephrectomy-clip) hypertension.
2. It was found that in response to a square-wave pressure step, the mean steady-state threshold pressure was 99.23 mmHg for the normotensive dogs and 12913 mmHg for the hypertensive dogs.
3. The degree of re-setting appeared to correlate with the duration of the hypertension but was not seen until an animal had been hypertensive for more than 4 days.
4. In one animal, hypertensive for 7 weeks but whose blood pressure returned towards normal after removal of the clip, the receptor characteristics were normal.
5. No change in the gain of the receptor sensitivity was seen.
6. Light-microscopy and electron miscroscopy of the opposite (non-perfused) carotid sinus revealed no differences in the fibre morphology or density between the hypertensive and normotensive animals (a random sampling, four normal and six hypertensive).