The aim of the present study was to investigate the protective role of pharmacological preconditioning on antioxidant enzymes using A(1) and A(3) adenosine receptor agonists in the recovery of the isolated myocardium after cardioplegic ischemia. Two different modes of preconditioning were studied: isolated rat hearts were perfused with A(1) receptor agonist 2-chloro-N(6)-cyclopentyladenosine (CCPA) or A(3) 2-chloro-N(6)-(3-iodobenzyl) adenosine-5'-N-methyluronamide (Cl-IB-MECA) (1 nM), followed by cardioplegic ischemia and reperfusion (30 min each) (perfusion mode), or CCPA or Cl-IB-MECA (100 micro g/kg) were injected intravenously 24 h before the experiment (injection mode). Hearts treated with CCPA improved in terms of mechanical function, infarct size, ATP levels, superoxide dismutase, and catalase (p < 0.005) in both modes of administration. Cl-IB-MECA was beneficial mainly in the injected group. Reduced damage to the mitochondria in the CCPA-treated hearts was observed using electron microscopy evaluation. In the Cl-IB-MECA-injected hearts, mitochondrial damage was moderate. CCPA in both modes of treatment and Cl-IB-MECA in the injected mode were beneficial in protecting the perfused isolated rat heart, subjected to normothermic cardioplegic ischemia. This protection was partially related to the higher myocardial activity of superoxide dismutase and catalase.