Hydrogen sulfide (H(2)S), produced by cystanthionine-γ-lysase (CSE) in the cardiovascular system, has been suggested to be the third gasotransmitter in addition to nitric oxide (NO) and carbon monoxide (CO). The present study aimed to investigate the role of H(2)S in ischemic postconditioning (IPO) during the early period of reperfusion. IPO with 6 episodes of 10 sec reperfusion followed by 6 episodes of 10 sec ischemia (IPO 2') was administered when reperfusion was initiated. Cardiodynamics and the concentration of H(2)S were measured at 1, 2, 3, 4, 5, 10, 20, 30, 60, 90 and 120 min of reperfusion. Lactate dehydrogenase (LDH) levels and infarct size were determined at the end of the reperfusion. The concentration of H(2)S was stable during the whole experiment in the control group, whereas it reached a peak at the first minute of reperfusion in the ischemia-reperfusion (IR) group. The concentration of H(2)S at the first minute of reperfusion in the IPO 2' group was higher compared to that of the IR group, which correlated with cardioprotection including improved heart contractile function and reduced infarct size and LDH levels. However, the above effects of IPO 2' were attenuated by pre-treatment with blockade of endogenous H(2)S production with DL-propargylglycine for 20 min prior to global ischemia. Furthermore, we found that other forms of IPO, IPO commencing at 1 min after reperfusion (delayed IPO) or lasting only for 1 min (IPO 1'), failed to increase the concentration of H(2)S and protect the myocardium. We conclude that the peak of endogenous H(2)S in the early reperfusion phase is the key to cardioprotection induced by IPO.