Heart preconditioning characterized by an induction of inducible nitric oxide synthase (iNOS) protein expression mediates cardioprotection from infarction. Myocardial infarction morbidity is known to increase in middle-aged men. The aim of this investigation is to determine whether iNOS protein expression is changed in ischemic hearts of aged male rats with or without preconditioning. 6-, 12- or 18-month-old male Wistar rats were used in the experiments. Preconditioning was induced by whole body hypoxia (10% O₂, 3 h) or whole body hyperthermia (40°C, 10 min). After 24 h, the animals were anesthetized; the hearts were excised and perfused according to the Langendorff mode. After initial stabilization, the isolated hearts were made totally ischemic (30 min) and then reperfused (40 min). The time of ischemic arrest, the time of postischemic heart recovery and the fatal arrhythmia events were registered to estimate a cardioprotection, induced by preconditioning. Protein expression was determined separately in the left and right ventricles of the hearts by Western blotting analysis using BioRad Labs protocols and equipment, and specific polyclonal antibodies from Sigma. The protein expression was estimated by computer densitometry using arbitrary units. It was shown that preconditioning caused induction of iNOS in myocardial tissues. In 6-month-old rats, the iNOS protein expression was 2-fold higher in right ventricles of the hearts than in the left ones. In aged rats (12- and 18-month-old), the expression of iNOS protein decreased 1.1- to 2.0-fold in comparison with 6-month-old rats. During postischemic reperfusion, the hearts of preconditioned 6-month-old rats demonstrated cardioprotective phenomena with limitation of unfavourable iNOS superinduction. However, in the hearts of aged rats was found marked individual differences in the cardioprotection and in the iNOS expression, from slight induction to strong activation like to control, non-preconditioned hearts. At the same time, the insufficient iNOS induction was accompanied by elevated induction of some heat shock proteins (HSP), especially, HSP32, which has iNOS-inhibiting properties. The data presented here have important implications for the validity of hypothesis that disturbances in myocardial induction of iNOS may represent the crucial event in the mechanisms leading to the rise of myocardial infarction events in aged men.