Plasma concentration of 11-dehydro thromboxane B2 (TxB2) is reported to be a reliable indicator of TxA2 production in human and rabbit. The purpose of this study is to examine whether we can use this parameter in dogs. The plasma concentration of 11-dehydro TxB2 was determined by enzyme immunoassay method using anti-11-dehydro TxB2 monoclonal antibody. When 10 micrograms/kg of TxB2 was intravenously administered to dogs in which endogenous production of prostanoids was blocked by aspirin, the plasma concentrations of 11-dehydro TxB2 before and 2, 5, 15, 60 min after TxB2 administration were 6.2 +/- 1.4, 54.2 +/- 5.0, 65.2 +/- 14.3, 65.2 +/- 16.4 and 31.0 +/- 7.5 pg/ml, while the corresponding TxB2 levels were 20.1 +/- 3.1, 3143.3 +/- 379.5, 1432.6 +/- 120.5, 356.8 +/- 38.9, and 128.9 +/- 13.6 pg/ml, respectively. Platelet-activating factor (PAF, 0.5 micrograms/kg) induced pulmonary vasoconstriction which was blocked by aspirin and OKY-046 (TxA2 synthetase inhibitor), indicating that TxA2 mediated this change. Plasma concentration of 11-dehydro TxB2 was increased by PAF. This change was also blocked by aspirin and OKY-046. These data suggest that the determination of plasma concentration of 11-dehydro TxB2 is useful to estimate the in vivo production of TxA2 in dogs.