Synthesis of prostaglandin I2, (PGI2, prostacyclin) by vascular endothelium (assayed by the ability of cultured endothelial cells to inhibit platelet aggregation) was inhibited by aspirin. At 100 mumol/l aspirin completely blocked measurable PGI2 production, but endothelial cells had substantially recovered their ability to synthesize PGI2 24 h after removal of the drug. In contrast, the effect of 1 mmol/l aspirin was still evident 24 h after drug withdrawal. Sulphinpyrazone also inhibited PGI2 synthesis, but was about 100 fold less potent than aspirin, and the effect of the drug was lost within 24 h of its addition, even when endothelial cells were left in contact with the drug during this period.