Doxorubicin (DOX=adriamycine), an effective chemotherapeutic agents for cancers, has severe cardiotoxicity. In the paresent study, we examined the protective effect of thermal preconditioning (TP) against apoptosis of rat cardiac muscle cells induced by DOX. Treatment with DOX (10 microM) for 24 hrs resulted in apoptosis of cardiac muscle cells, which was evaluated by examining "DNA ladder" formation and TUNEL staining. The number of TUNEL-positive cells was significantly decreased in cells subjected to TP by incubation at 42 degrees C for 30 min, 24 hrs prior to DOX-treatment. Antisense oligonucleotides of the heat shock protein (HSP) 70 blunted this effect. These results indicate that DOX-induced apoptosis in cardiac muscle cells is prevented by TP, at least in part, via a HSP70-mediated mechanism.