1. The changes in R--R heart interval that result from step-increase and step-decrease in carotid sinus transmural pressure induced by a variable pressure neck chamber were measured in seven normal men. Observations were made at rest, and during isometric hand-grip exercise at 24%, 44% and 64% of maximal voluntary contraction. 2. The response of heart interval to increase in carotid sinus transmural pressure was progressively and markedly diminished according to the strength of hand-grip. This effect was fully developed from the moment of onset of the exertion. 3. The response of heart interval to decrease in carotid sinus transmural pressure was much less consistently affected by hand-grip exercise.

1. Two methods were used for altering blood pressure in conscious rabbits by up to & 30 mmHg from the resting level in order to characterize the baroreceptor-heart rate reflex. These were to inflate descending aortic or inferior vena caval cuffs, or to give brief intravenous infusions of phenylephrine or glycerol trinitrate. The relation of change in blood pressure to change in heart interval was examined, both during the initial 'ramp' changes of these variables and when they had reached a 'steady-state'.2. Both methods allowed the construction of 'steady-state' sigmoid stimulusresponse curves whose parameters were reproducible within animals, and which were attended by a relatively small variance between animals. The inflatablecuff method gave a higher average value for maximum gain than the vasoactive drug method (10.0 u 5.4 ms/mmHg) and a narrower pressure range between the threshold and saturation points of the response, but values for the other parameters were similar. Corresponding parameters obtained by the two methods correlated closely.3. After denervating all arterial baroreceptors except one carotid sinus, 'steady-state' maximum gain by cuff-inflation was reduced to 3.1 ms/mmHg, and to 2.6 ms/mmHg by the vasoactive drug method. The heart interval range between upper and lower plateau levels was reduced, but the pressure range between threshold and saturation points was widened, with both methods. 4. During the initial 'ramp' changes of blood pressure the sensitivity of the reflex was described by the slope of the linear regression of heart interval on 'mean blood pressure. The slopes obtained by aortic cuff inflation, and by infusion of either vasoactive drug, correlated positively with 'steady-state' maximum gain. However, the reproducibility of the 'ramp' method was inferior to that of the 'steady-state' method with respect to reflex sensitivity, and other parameters of the stimulus-response relation cannot be estimated. I. B. Faris et al.5. Complete baroreceptor denervation virtually eliminated heart rate changes over the range of blood pressures usually employed. However, when blood pressure was increased by more than 40 mmHg a profound reflex bradycardia and hypopnoea were then evoked.

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1. The change in arterial pressure and heart rate resulting from alteration of carotid sinus transmural pressure by a median--34 mmHg and +33 mmHg by means of a variable-pressure neck chamber was tested in seven male volunteer subjects, at rest and during exertion of 35, 45 and 65% of maximum voluntary handgrip. 2. During 60 s of 35 and 45%, and during 30 s of 65%, of maximal voluntary handgrip there was virtually no alteration of the response of blood pressure to alteration carotid sinus transmural pressure. 3. The bradycardic response to increase in carotid sinus transmural pressure was reduced at various times after the commencement of handgrip at 45 and 65% of maximum voluntary contraction. 4. It is concluded that a reduction in arterial baroreceptor reflex sensitivy does not play an important role in the initiation of the increase in arterial blood pressure and heart rate caused by isometric exercise. 5. The hypothesis is advanced that some of the cardiovascular changes in exercise may result from elevation of the central 'set point' for blood pressure.

SUMMARY1. A method is described for altering the pressure across the wall of the carotid sinus in conscious rabbits by enclosing the carotid bifurcation in a rigid, fluid-filled capsule. The extracapsular arterial baroreceptors were denervated.2. The baroreceptor-heart rate reflex, elicited by injecting vasoactive drugs or inflating aortic and vena caval cuffs, was used to test the new method. The function of the carotid sinus was shown to be unaffected by enclosure in the capsule. Denervation of the extracapsular baroreceptors reduced the gain of the baroreceptor-heart rate reflex two-to threefold.3. The characteristics of the carotid baroreceptor reflex were studied in sixteen animals by the capsule method. Median estimates of maximum gain, and the range over which blood pressure changed, were 1 1 mmHg/mmHg and 57 mmHg respectively. There was good agreement between duplicate estimates made 1-20 days apart.4. There was only a weak association between the effects on blood pressure and heart rate of altering carotid sinus transmural pressure. Autonomic blockade of the heart, so that its rate was fixed, did not reduce the gain or range of blood pressure change.