Myocardial ischemia is a complex process leading to the simultaneous release of a number of mediators, including thromboxane A₂ (TxA₂) and bradykinin (BK), that activate cardiac spinal afferents. The present study tested the hypothesis that TxA₂ and BK reciprocally interact to excite ischemically sensitive cardiac afferents. Nerve activity of single cardiac afferent units was recorded from the left sympathetic chain or rami communicantes (T2 - T5) of anesthetized cats. Fifty two ischemically sensitive afferents (CV= 0.27 - 3.35 m/s, 7 A- and 45 C-fibers) were identified. Repeated injection (1 μg) of BK, into left atrium (LA) 4 min after administration of U46619 (5 μg, LA), a TxA₂ mimetic, induced a significantly larger cardiac afferent response than the first response to BK (0.61±0.14 to 1.95±0.29 vs.0.66±0.09 to 2.75±0.34 imp/s, first vs. second injection, n=8). Conversely, blockade of TxA₂ receptors with BM13,177 (30 mg/kg, iv) attenuated the responses of eight other afferents to BK (1 μg, LA) by 45%. In contrast, repeated BK (1 μg, LA) induced consistent discharge activity in six separate afferents. Co-administration of U46619 (5 μg) and BK (1 μg, LA) together caused a total response that was significantly higher than predicted response by simple addition of the individual responses. BK (1 μg) facilitated eight cardiac afferents response to U46619 (5 μg, LA) by 64%. In contrast, repeated U46619 without intervening BK stimulation evoked consistent responses in seven other ischemically sensitive afferents. Lastly, inhibition of cyclooxygenase with indomethacin (5 mg/kg, iv) eliminated the potentiating effects of BK on the cardiac afferent response to U46619 (5 μg, LA) but did not alter the afferent response to U46619. These data suggest that BK and TxA₂ reciprocally interact to stimulate ischemically sensitive cardiac afferent endings leading to synergistic afferent responses and that the BK sensitization effect is mediated by cyclooxygenase products.