The blood pressure and heart rate responses to the Valsalva maneuver as measured during cardiac catheterization are a reliable index of cardiovascular integrity. The present study indicates that heart rate itself, as monitored by the electrocardiograph, is an equally reliable index of the dynamic changes occurring during and after straining. In a normal group of subjects the heart rate slowed with the onset of straining (phase 1), increased during (phase 2) and after straining (phase 3), and was followed by a marked bradycardia (phase 4). These responses occurred to a significantly lesser degree in subjects with advanced cardiac disease, the absense of the bradycardia in phase 4 being of the greatest statistical importance. Using these criteria, subjects with cardiovascular abnormalities of uncertain dynamic significance were separated into clinically valid groups of reactors and nonreactors.