1. We studied the effect of oral administration of acetylsalicylic acid (1200 mg/day for 3 days) on the urinary excretion of 6-ketoprostaglandin F in normal human subjects as an index of prostacyclin production in vivo.

2. The concentrations and excretion rate in urine fell to 45% of pretreatment levels in 3 days, but returned to pretreatment values after 7 days.

3. These results suggest that production of prostacyclin in vivo is only partially inhibited by high doses of aspirin and that there are sites of production of prostacyclin which are protected from inhibition by aspirin and which contribute to urinary 6-ketoprostaglandin F. The measurement of 6-ketoprostaglandin F in urine may therefore be of only limited value as an index of the metabolism of vascular tissue in vivo.

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