The possible modulation of adenosine-induced relaxation by endogenous prostaglandins was studied in isolated porcine, bovine and canine coronary arteries. Artery rings were mounted for isometric tension recording in organ bath filled with Krebs-Ringer-bicarbonate solution (37 °C). Increasing concentrations (10<sup>-6</sup>–10<sup>–4</sup> M) of adenosine relaxed contractions induced by 35 mM KC1. The cyclo-oxygenase inhibitor indomethacin (5 × 10<sup>–6</sup> M) significantly augmented the relaxations in response to adenosine in coronary arteries isolated from all three species. In the porcine coronary artery (which was most sensitive to exogenous adenosine) the mixed lipoxygenase/cyclo-oxygenase inhibitor phenidone augmented, while exogenous arachidonic acid (3 × 10<sup>–5</sup> M) or a decrease in bath O<sub>2</sub>-level from 95 to 20% depressed adenosine-induced relaxations. Indomethacin completely prevented the inhibitory effect of decreasing bath O<sub>2</sub>, but only partially antagonized the suppression caused by arachidonic acid. The data demonstrate that arachidonic acid and its cyclo-oxygenase metabolites modulate (depress) adenosine responsiveness in isolated coronary arteries.