The influence of daltroban (BM13.505; SK&F 96148), a thromboxane (Tx) A2-receptor-blocking agent, on responses to the TxA2 mimics U-46619 and U-44069 was investigated in the pulmonary vascular bed of the intact-chest cat under constant-flow conditions. Daltroban (5 mg/kg iv) had no significant effect on mean baseline vascular pressures but significantly decreased responses to the TxA2 mimics without altering responses to prostaglandin (PG) F2 alpha or PGD2 or the PGD2 metabolite 9 alpha, 11 beta-PGF2. Dose-response curves for U-46619 and U-44069 were shifted to the right in a parallel manner, and daltroban had no significant effect on responses to norepinephrine, serotonin, angiotensin II, BAY K 8644, endothelin-(ET) 1, ET-2, or platelet-activating factor (PAF). After administration of daltroban, responses to U-46619 returned to 50% of control in 90 min and responses to the PG and TxA2 precursor arachidonic acid were decreased significantly. These results suggest that daltroban selectively antagonizes TxA2-receptor-mediated responses in a competitive and reversible manner. These data provide support for the hypothesis that discrete TxA2 receptors unrelated to receptors stimulated by PGF2 alpha, PGD2, or 9 alpha, 11 beta-PGF2 are present in the pulmonary vascular bed of the cat. The present data suggest that pulmonary vasoconstrictor responses to PAF and ET peptides are not dependent on activation of TxA2 receptors in the cat.