1. A method is described for measuring the urinary excretion of 6-keto-prostaglandin F1α, the stable hydrolysis product of prostaglandin I2, by stable isotope dilution gas chromatography–mass spectrometry.
2. Three different doses of prostaglandin I2 were infused intravenously into conscious dogs and the effects on systemic and renal haemodynamics and urinary sodium excretion were observed.
3. The two highest infusion rates of prostaglandin I2 (15 and 30 ng min−1 kg−1 body weight) induced significant decreases in systematic blood pressure and dose-related increases in sodium excretion, but no change in renal haemodynamics.
4. There was a linear relationship between urinary excretion of 6-keto-prostaglandin F1α and the rate of infusion of prostaglandin I2.
5. The calculated basal rate of entry of prostaglandin I2 into the systematic circulation in conscious dogs is 4 ng min−1 kg−1 body weight, which is substantially higher than that previously reported in man.