Previous studies have shown that rabbit hearts subjected to in vivo left ventricular myocardial infarction and subsequent ex vivo perfusion respond to A'-formylmethionyl-leucylphenylalanine (fMLP) with enhanced eicosanoid synthesis. This synthetic response occurs primarily in the right cardiac atrium, a site distant from the injury, and is not the result of increased enzymatic capacity for arachidonate metabolism. To further investigate the mechanism of this enhanced synthetic response, [3H]fMLP binding was characterized and binding sites were localized autoradiographically in intact tissue sections prepared from control hearts and hearts subjected to left ventricular myocardial infarction (1, 2, and 4 days postinfarction). Analysis of binding isotherms revealed a saturable high affinity (KD∼l nM) fMLP binding site in the right cardiac atrium. After myocardial infarction specific binding in right atria (2 day) increased 12-fold (B=14.8 ± 2.4 fmol/cm2) compared with normal controls (Bnu=1.2 ± 0.1 finol/cm2). Specific binding of fMLP also increased in the infarcted zone of left ventricle, but B-u was only 30–50% that of right atria. Light microscopic autoradiography studies revealed that atrial fMLP binding sites were highly concentrated in small morphologically undifferentiated cells located in interstitial and perivascular spaces. These results demonstrate the existence of a formylated peptide receptor on a nonleukocytic cell and illustrate its regulation following left ventricular injury. {Circulation Research 1989;65:215–223)