Nitric oxide (NO) has been known to play various functional and pathological roles as an intracellular or intercellular messenger in the heart. In this study, we investigated whether NO produced during ischemia was involved in the coordination of ATP supply and demand, and also in protection from cell death using cultured cardiac myocytes. Unexpectedly, the survival rate of myocytes for 3 h simulated ischemia (SI) was increased as compared with that for 2 h SI at 24 h after reperfusion. The cellular ATP level at 3 h after the start of SI was increased compared with that at 2 h, and was almost the same as that before the start of SI. The cellular ATP level at 3 h SI was significantly reduced by either the inhibition of nitric oxide synthase (NOS) or scavenging of NO. Either the inhibition of NOS or the scavenging of NO during SI for 3 h also resulted in a significant decrease in the survival rate of myocytes.Immunocytochemical and Western blot analyses revealed that the expression of nNOS was most evident in cardiac myocytes, but no significant change was observed in the expression of all three NOS isoforms at 2 h SI and at 3 h SI. The fluorescent intensity of DAF-FM was significantly increased at 3 h SI as compared with that at 2 h SI, and 2 the increase in DAF fluorescence during SI was almost completely suppressed by treatment with L-VNIO, a potent specific inhibitor of nNOS. In addition, treatment with L-VNIO decreased the cellular ATP level and survival rate. This study suggested that the enhanced production of NO was critical in balancing ATP supply and demand during ischemia, and also in protecting cells from ischemia/reperfusion injury.