Hyperventilation is an important feature of panic disorder, and an association has been reported between panic disorder and hypertension. We have examined the effect of hyperventilation on the blood pressure (BP) of healthy subjects. Twenty six subjects were randomised in a balanced two-period cross-over study to compare the effects of hyperventilation with that of normal breathing on sitting BP, heart rate and the electrocardiogram. Each study phase lasted 40 min, with 15 min of baseline observation, 5 min of hyperventilation or normal breathing, and 20 min of continued observation. Hyperventilation significantly increased SBP by 8.9 mmHg (95% CI 3.8-13.8, P < 0.01), diastolic blood pressure by 8.2 mmHg (95% CI 1.7-14.7, P < 0.05), mean arterial pressure by 10.0 mm Hg (95% CI 3.3-16.7, P < 0.01) and heart rate by 36 beats/min (95% CI 31-44, P < 0.01). The changes in diastolic and mean arterial pressure correlated significantly with the total volume of air expired during hyperventilation (r = 0.57, p < 0.01 and r = 0.50 P < 0.01, respectively), but not with the change in expired carbon dioxide. In the electrocardiogram, T wave changes occurred in the inferior leads in 10 of 26 subjects, but there were no significant changes in other measurements. Hyperventilation significantly increased the BP of healthy subjects, and the role of hyperventilation in the link between panic disorder and hypertension deserves further study.