The cardioprotective effect of opioids or glycogen synthase kinase (GSK) inhibitors given at reperfusion has not been investigated in diabetes models. Therefore, nondiabetic (NDBR) or streptozotocin-induced diabetic (DBR) rat hearts were subjected to 30 min of ischemia and 2 h of reperfusion. Groups of NDBR or DBR were administered either vehicle, morphine (0.3 mg/kg), or the GSK inhibitor SB216763 (0.6 mg/kg) 5 min before reperfusion. SB216763 (but not morphine) reduced infarct size in DBRs (44 +/- 1* and 55 +/- 2%, respectively), while both agents reduced infarct size in NDBRs versus untreated NDBRs or DBRs (44 +/- 3*, 42 +/- 3*, 60 +/- 2, and 56 +/- 2%, respectively, *P < 0.001). Morphine-induced phospho- (P-)GSK3beta was reduced 5 min after reperfusion in DBRs compared with NDBRs (0.83 +/- 0.29 and 1.94 +/- 0.12 [P < 0.05] pg/microg tissue, respectively). The GSK3beta mediators, P-Akt, P-extracellular signal-related kinase (ERK)1, and P-signal transducer and activator of transcription (STAT)3, were also significantly reduced in untreated DBR compared with NDBR rats. Morphine-induced elevations of P-Akt, P-ERK1, P-p70s6, P-janus-activated kinase-2, and P-STAT3 in NDBRs were also blunted in DBRs. H9C2 cells raised in 25 mmol/l compared with 5.56 mmol/l glucose media also demonstrated reduced morphine-induced P-GSK3beta, P-Akt, P-STAT3, and P-ERK1 after 15 min. Hence, acute GSK inhibition may provide a novel therapeutic strategy for diabetic patients during an acute myocardial infarction, whereas morphine is less effective due to signaling events that adversely affect GSK3beta.