Apelin 12 (A-12) was synthesized by the automatic solid phase method with the use of Fmoc technology. The synthesized peptide was purified by preparative HPLC and identified by 1H-NMR spectroscopy and mass spectrometry. Acute myocardial infarction was induced by 40-min LAD occlusion followed by 60-min reperfusion in narcotized Wistar rats. A-12 was administrated at the onset of the reperfusion at doses of 0.07, 0.35 and 0.70 micromole/kg; N(G)-nitro-L-arginine methyl ester (L-NAME), a NOS inhibitor, was applied at a dose of 10 mg/kg 10 min prior to reperfusion alone or before A-12 administration (0.35 micromole/kg); saline was used in control. The indicated A-12 doses induced a transient reduction of the arterial systolic blood pressure (ASBP) to 85, 58, and 56% of the initial level, respectively, which was accompanied by its recovery by the end of reperfusion. All A-12 doses significantly limited myocardial infarct size by 26, 40 and 33%, respectively, compared to the value in control. After administration of A-12 at dose of 0.35 micromol/kg, this effect was combined with reduction of MB-creatine kinase (MB-CK) and lactate dehydrogenase (LDH) activities in plasma at the end of reperfusion by 56 and 47%, respectively, compared to the values in control. Inhibition of NO formation by L-NAME increased SABP but did not affect myocardial infarct size compared with that in control. Coadministration of L-NAME and A-12 resulted in lesser reduction of ASBP during reperfusion than injection of A-12 alone. This intervention led to an increase in infarct size by 26% with concomitant 1.8- and 1.5-times elevation of MB-CK and LDH activities, respectively, compared to the values in the A-12 group. The results indicate that NO is involved as a mediator of the effects of A-12 on the overall protection consisting in a limitation of infarct size and reduction of postischemic cardiomyocyte membrane damage. Cardioprotective mechanisms of apelin action are discussed.