Vulnerability of aged hearts to ischemia may be due to defects in protective mechanisms provided by heat shock proteins (HSPs). To determine whether there is a defect in the induction of HSPs by ischemia in old hearts, HSP72 and HSP73 (inducible and constitutive HSP70, respectively) mRNA induction was examined in young (2-mo-old; n = 36) and old (18-mo-old; n = 32) rat hearts. Transient (10- or 20-min) ischemia was applied by tightening a snare placed around left coronary arterial branches 3 days before examination to avoid the effect of operation on induction. HSP72 mRNA was induced markedly in young hearts after 10-min ischemia, peaked at 2 h, but was induced only slightly in old hearts. HSP73 mRNA was also induced in young hearts, peaked at 4 h, but was not induced in old hearts. The mRNAs were markedly induced in old hearts as well after 20-min ischemia, which was accompanied by the induction of HSP72 protein. Thus the age-related modulation of HSP72 and HSP73 mRNAs suggests a defective sensing mechanism for ischemia in old hearts.