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Figure 3 Mean inhibition of platelet cyclooxygenase-1 activity ex vivo, as assessed by the measurement of serum thromboxane (TX)B₂ levels (A), arachidonic acid-induced platelet aggregation ex vivo (B), and TX biosynthesis in vivo, as assessed by the measurement of urinary 11-dehydro-TXB₂ levels (C), in subjects taking low-dose aspirin alone (100 mg daily) for six days (hatched bars) and then readministered with naproxen (500 mg twice daily, with the first dose administered 2 h after aspirin) for further six days (open bars). The solid bars show the effects of the same medications administered in reversed order for further six days after a washout period of 14 days. Values are reported as mean ± SEM, n = 4. All times are hours after the administration of the first study drug. Open bars = aspirin before naproxen (twice daily); solid bars = naproxen (twice daily) before aspirin; hatched bars = aspirin.