In unanesthetized humans during voluntary hyperventilation, hypocapnia causes coronary vasoconstriction, decreased oxygen (O₂) supply and availability to the heart. This can induce local epicardial coronary artery spasm in susceptible patients. Its diagnostic potential for detection of early heart disease is unclear. This is because such hypocapnia produces an inconsistent and unreproducible effect on electrocardiogram (ECG) in healthy subjects. To resolve this inconsistency we have applied two new experimental techniques in normal healthy subject to measure the effects of hypocapnia on their ECG; mechanical hyperventilation and averaging of multiple ECG cycles. In 15 normal subjects we show that hypocapnia (20 ±1 mmHg) significantly reduced mean T wave amplitude by 0.1 ±0.0 mV. Hypocapnia also increased mean heart rate by 4 beats.min^-1 without significantly altering blood pressure, ionized calcium or potassium levels, nor the R wave, nor other features of the ECG. We therefore provide the first unequivocal demonstration that hypocapnia does consistently reduce T wave amplitude in normal, healthy subjects.