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The amplitude of respiratory sinus arrhythmia (RSA) while breathing quietly was measured in 32 mildly hypertensive subjects and in 28 normotensive controls. In 35 subjects, the RSA amplitude in response to rebreathing carbon dioxide was also measured. The RSA amplitude while breathing quietly was smaller in the hypertensive group, particularly in the subjects having the highest levels of diastolic blood pressure. The breathing rate was greater and the heart period (R–R interval) shorter in the hypertensive subjects. After adjusting for differences in the breathing rate, the RSA amplitude was similar in both groups, but the heart period remained shorter in the hypertensive subjects. These findings suggest that breathing style may contribute to a reduction in parasympathetic cardiac control in mild hypertension.