The opening of unapposed connexin 43 hemichannels (Cx43Hc) under ischemic stress leads to cell death and irreversible tissue injury. Here, we investigate for the first time in vivo the cardioprotective potentials of two unique Cx43 structural-mimetic peptides (Cx43MPs) presumed specific blockers of Cx43Hc, Gap26 and Gap27, when injected intravenously using a rat model of myocardial infarction.Sprague Dawley rats were utilized. Myocardial infarction was induced by occluding the left anterior descending coronary for 40min followed by 2days of reperfusion. Interestingly, single bolus injections of Gap26 or Gap27 (1μg/kg) into the jugular vein caused infarct size reductions by up to 61% with reference to control rats injected with saline at similar timings. Infarct reductions did not vary significantly whether peptides were administered before or after the onset of ischemia. Although the two peptides allegedly interact with distinct structures of Cx43, co-administration of Gap26/Gap27 in equal doses did not confer additive protection to hearts (maximum infarct reduction by 64%). Using patch clamp technique, we provide unique and direct evidence for the inhibitory effect of Cx43MPs on genuine human Cx43Hc transiently expressed in the ion channel-deficient tsA201 cells. In concordance with the cardioprotective effect observed in vivo, co-application of both peptides did not cause cumulative current inhibition. A safety profile of Cx43MPs was also addressed.Our results reveal great therapeutic potential of Cx43MPs in treatment of myocardial infarction. Their practical way and timing of administration and their apparent safe profile make them promising tools to fight ischemic heart disease.