Recently, total chemical synthesis of thromboxane was achieved. The in vitro activity of synthetic thromboxane A2 is indistinguishable from biologically generated material. The present study describes the in vivo characterization of synthetic thromboxane A2 on the regional blood flow distribution of the canine heart. Local injections of synthetic thromboxane A2 into the coronary vasculature caused marked reductions in coronary blood flow, measured by both radiolabeled microsphere injection and an electromagnetic flow device. The threshold concentration required to bring about this effect varied greatly between dogs and ranged from 0.125 microgram/kg to 2.0 micrograms/kg. Similarly, the dose of thromboxane A2 required to aggregate dog platelets in vitro varied from 30 ng/ml to 1,000 ng/ml. Bolus injections of 2 micrograms/ml thromboxane A2 into the circumflex or left anterior coronary artery resulted in a simultaneous reduction in platelet count in coronary sinus blood of 83 +/- 5.2% (mean +/- SEM, n = 4, p = .0005). Both flow reduction and platelet effects were transient and localized. The time taken from onset to recovery of the response to control levels was 77 +/- 6.0 seconds (mean +/- SEM) for flow and 70-80 seconds for platelet count. Injections of thromboxane A2 caused a small but significant increase in heart rate with no change in systemic blood pressure. In conclusion, the in vivo actions of synthetic thromboxane A2 are consistent with the vasoconstrictor and platelet aggregatory effects seen in vitro, but dogs vary considerably in their sensitivity.