1. The biotransformation of arachidonic acid and prostacyclin in the circulation was studied in anaesthetized dogs, using the blood-bathed organ technique.
2. In passage through the lungs, arachidonate (50–800 μg kg−1 min−1) was transformed into prostacyclin. No thromboxane A2 or prostaglandin E2 could be detected in arterial blood.
3. In dogs treated with indomethacin (5 mg/kg), intravenous infusions of arachidonate had no cardiovascular effects and no prostacyclin was produced. Therefore, the vasodilator effects of arachidonate in vivo may be attributable to prostacyclin formation.
4. Prostacyclin, unlike prostaglandin E2, is not inactivated by passage across the lungs, and only about 50% disappears in one passage through peripheral vascular beds.
5. Thus prostacyclin released from the lungs could function as a circulating vasodilator and contribute to the regulation of blood vessel tone and blood pressure.