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The effects of the intravenous (i.v.) administration of 1.1 mg/kg of flunixin meglumine on thromboxane B2 (TxB2) concentrations were studied in sedentary and 2-year-old horses in training. The baseline TxB2 serum concentrations generated during clotting were 2.89 ± 81, 2.19 ± 0.25 and 0.88 ± 0.12 ng/ml for the 2-year-old Thoroughbreds in training, sedentary horses under 10 and over 10 years old, respectively. There was a significant difference in baseline TxB2 concentrations between older and younger horses (P < 0.005). Significant reduction in TxB2 production from baseline were noted at 1 (P < 0.01) and 4 h (P < 0.01) but not at 8 h after flunixin administration. The percent reduction in serum TxB2 concentration at 1 h after the administration of flunixin was 68.6 ± 7.8 and 45.2 ± 6.8 for the training and sedentary horses, respectively; the differences were significant (P < 0.04). Serum concentrations of TxB2 returned to baseline values by 12–16 h after flunixin administration. The results of this study indicate a difference in the TxB2 concentrations of older vs. younger horses and a difference in the suppression of TxB2 after the administration of flunixin in 2-year-old Thoroughbreds in training compared to sedentary horses. The results of this study suggest that the detection of low concentrations of flunixin in urine 24 h post-administration may not represent pharmacologic effective concentrations of flunixin in plasma.