The aim of the present study was to investigate the mechanism of effect of 3-nitropropionic acid- (3-NP) induced late preconditioning in rat heart. For this purpose 20-30 min before 3-NP (20 mg/kg, i.p.) injection, the rats were treated intraperitoneally with 5-hydroxydecanoate (40 mg/kg, 5-HD, mitochondrial $K_{ATP}$-channel blocker), L-NAME (100 mg/kg, NOS inhibitor), N-2-mercaptopropionylglycine (100 mg/kg, MPG, free radical scavenger), or superoxide dismutase+catalase (10000+10000 IU/kg, SOD+CAT). Control rats received saline only without 3-NP pretreatment. After two days, hearts were isolated and perfused at a constant pressure in a Langendorff apparatus. 15-min global ischemia followed by 30-min reperfusion was applied to all hearts. Pretreatment of 3-NP significantly reduced infarct size, creatine kinase-MB (CK-MB), lactate dehydrogenase (LDH) levels, and incidence of ventricular tachycardia (VT) compared with the control group receiving saline only. 5-HD, L-NAME, MPG, or SOD+CAT treatment statistically reversed 3-NP-induced reduction in infarct size. Although CK-MB, LDH levels, and incidence of VT were also reduced by L-NAME, MPG, or SOD+CAT treatment, only 5-HD significantly inhibited beneficial effects of 3-NP on all of the parameters above. These results showed that mito-$K_{ATP}$ channels play a pivotal role in late preconditioning effect of 3-NP in the isolated rat heart. However, other mediators such as reactive oxygen species and NO may be, at least in part, involved in mechanisms of this effect.