Previous reports have demonstrated an antagonistic influence of adenosine in the biological effects of prostaglandins (PGs). We examined such a possible relationship on the isolated rat heart perfused at constant pressure and maintained at a constant heart rate. PGE2 and PGF2 alpha (2.8 X 10(-11) to 2.8 X 10(-7) M) exerted a positive inotropic influence on the heart with PGF2 alpha demonstrating the greater maximum effect. PGI2 had a negative inotropic effect only at high concentrations (2.8 X 10(-9) to 2.8 X 10(-7) M). The coronary flow was decreased by both PGE2 and PGF2 alpha. The concentrations of PGI2 employed had minimal vasoactive properties (20% increase in flow at 2.9 X 10(-7) M). Adenosine at concentrations which increased coronary flow by 24 to 63% (0.1 to 10 microM) had no influence on either the alteration in contractile force produced by PGE2 PGF2 alpha, or PGI2 or on the coronary effects of PGE2 or PGI2. Adenosine did, however, attenuate the degree of coronary flow reduction produced by PGF2 alpha administration. These results are suggestive of a selective inhibiting influence of adenosine on the coronary constricting effects of PGF2 alpha, in the isolated rat heart.