1. Serial studies of the baroreflex control of heart rate (baroreflex sensitivity) were made in thirty-two patients on long-term haemodialysis by relating the reflex slowing of the heart to the arterial pressure rise produced by repeated sudden injections of phenylephrine.
2. Baroreflex sensitivity was less in the older patients and in those with higher pressures. The response was about 50% of that found previously for other subjects of comparable ages and arterial pressures without renal failure.
3. Haemodialysis improved reflex sensitivity over the long term, but did not have any consistent immediate effect.
4. Patients who had malignant hypertension in the past had lower reflex sensitivities than others (with similar ages and pressures at the time of study) whose blood pressures had never been raised, but there were individual exceptions.
5. Reflex sensitivity changed very little with different blood pressures in any one subject.
6. Three patients in whom hypertension was thought to have preceded renal failure had reflex sensitivities similar to eleven others in whom the hypertension was secondary to renal disease.
7. Reflex sensitivity was no different in three patients whose blood pressure could be controlled only by bilateral nephrectomy from that in others in whom the pressure could be controlled by salt restriction.
8. Reflex sensitivity was slightly reduced in two patients studied in the diuretic phase of acute renal failure; it was markedly reduced in a third who was also pregnant.