Dynamin-related protein-1 (DRP-1) dependent mitochondrial fission may influence cardiac tolerance to ischemic or oxidative stress, presenting a potential 'cardioprotective' target. Effects of dynamin inhibitors MDIVI-1 and dynasore on injury, mitochondrial function and signaling proteins were assessed in distinct models: ischemia-reperfusion (I-R) in mouse hearts, and oxidative stress in rat H9c2 cardiomyoblasts. Hearts exhibited substantial cell death (~40 IU LDH efflux) and dysfunction (~40 mmHg diastolic pressure, ~40% contractile recovery) following 25 min ischemia. Pre-treatment with 1 µM MDIVI-1 reduced dysfunction (30 mmHg diastolic pressure, ~55% recovery) and delayed without reducing overall cell death, whereas 5 µM MDIVI-1 reduced overall death while paradoxically exaggerating dysfunction. Post-ischemic expression of mitochondrial DRP-1 and phospho-activation of ERK1/2 were reduced by MDIVI-1. Conversely, 1 µM dynasore worsened cell death and reduced non-mitochondrial DRP-1. Post-ischemic respiratory fluxes were unaltered by MDIVI-1, although a 50% fall in complex-I flux control ratio was reversed. In H9c2 myoblasts stressed with 400 µM H2O2, treatment with 50 µM MDIVI-1 preserved metabolic (MTT assay) and mitochondrial (basal respiration) function without influencing survival. This was associated with differential signaling responses, including reduced early vs. increased late phospho-activation of ERK1/2, increased phospho-activation of AKT, and differential changes in determinants of autophagy (reduced LC3B-II/I vs. increased Parkin) and apoptosis (reduced PARP cleavage vs. increased BAX:BCL2). These data show MDIVI-1 (not dynasore) confers some benefit during I-R/oxidative stress. However, despite mitochondrial and metabolic preservation, MDIVI-1 exerts mixed effects on cell death vs. dysfunction, potentially reflecting differential changes in survival kinase, autophagy and apoptosis pathways.