The effect of atrial pacing on intracoronary thromboxane production was investigated in 35 patients with stable (n = 19) or unstable (n = 16) angina. Arterial and coronary sinus thromboxane B2, the stable metabolite of thromboxane A2, myocardial lactate extraction and thermodilution coronary sinus flow were measured before, during and immediately after atrial pacing until the onset of angina. Pacing did not significantly increase coronary sinus thromboxane B2(rest, 233 ± 107 pg/ml; pacing, 249 ± 154 pg/ml; postpacing, 330 ± 309 pg/ml) (mean ± standard deviation) despite a moderate increase in arterial thromboxane B2(rest, 270 ± 170 pg/ ml; pacing, 387 ± 364 pg/ml; postpacing, 446 ± 420 pg/ ml) (all changes probability [p] < 0.05). A positive trans-myocardial thromboxane B2gradient, suggesting intracoronary thromboxane A2production, occurred in only five patients at rest (gradient = 60 ± 35 pg/ml). During pacing, a transmyocardial thromboxane B2gradient was not observed despite myocardial lactate production in 18 patients. A postpacing gradient was observed in eight patients (gradient = 284 ± 349 pg/ml). These gradients were significantly more frequent in patients who produced lactate during pacing (7 of 18) than in patients without lactate production (1 of 17) (p < 0.05). In patients with and without a postpacing gradient, coronary vascular resistance decreased with pacing and returned to rest levels immediately after pacing, suggesting that a post-pacing thromboxane gradient does not significantly alter coronary tone.