1. In six healthy subjects the amplitude and phase of respiratory sinus arrhythmia were determined at five different respiratory cycle lengths ranging from 3 to 9.5 s.
2. At each respiratory cycle length the carotid baroreceptor-heart rate reflex response was determined by cyclical neck suction at −40 mmHg at five different cycle lengths covering the same range of 3–9.5 s.
3. The application of cyclical neck suction increased the amplitude of respiratory sinus arrhythmia in all but the longest respiratory cycle lengths.
4. With increasing respiratory cycle length the amplitude of sinus arrhythmia increased, and R-R intervals were at their longest at an earlier phase of the respiratory cycle. Similarly, with increasing suction cycle length the amplitude of the cardiac interval response increased and the phase angle decreased.
5. The cardiac interval responses to respiration and to neck suction at different frequencies were independent of each other, the heart rate at any moment resulting from the algebraic summation of the two responses.