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.