1. Indomethacin, an inhibitor of the cyclo-oxygenase system that converts arachidonic acid into prostaglandins and related substances, was infused intravenously in 12 healthy volunteer subjects.

2. Systemic systolic and diastolic blood pressures and heart rate were recorded in all subjects, and in most of them also the systemic arteriovenous oxygen difference, the total oxygen uptake and the pulmonary arterial and wedge pressures.

3. The infusion of indomethacin was followed by a decreased cardiac output (from 7·3 ± 0·3 to 6·3 ± 0·3 litres/min) and an increased mean systemic blood pressure (from 92 ± 1 to 102 ± 1 mmHg), indicating an elevation of the total systemic vascular resistance (from 98 ± 4 to 124 ± 5 kPa)l−1 s) by indomethacin. The ventilation and the pulmonary vascular resistance did not change after the infusion of indomethacin.

4. The results suggest that products formed by the cyclo-oxygenase system at rest exert a relaxing effect in certain parts of the systemic vascular bed, thereby lowering the systemic vascular resistance.

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