The aim of this study was to define the characteristics of heart rates and myocardial changes in rats exposed to carbon monoxide (CO), and the effects of reoxygenation, atenolol (a beta-blocker) and melatonin after sublethal CO intoxication. Widespread use of beta-blockers in cardiology practice and growing literature on the positive effect of melatonin in ischaemia reperfusion lead us to question their effects in case of CO intoxication. Rats were exposed to CO. After sublethal intoxication the rats were reoxygenated with ambient air. Subsequently blood values, electrocardiographic recordings and pathological changes were examined for each groups. Five rats died after CO intoxication in the control group: no myocardial changes were seen in light microscopy However, myocardium of seven reoxygenated rats presented contraction bands. Seven reoxygenated rats pretreated with atenotol had a higher number of contraction bands of myocardial cells. Seven reoxygenated rats pretreated with melatonin had more contraction bands than reoxygenated rats. and heart rate recordings of these animals revealed a profund and sustained bradycardia. Thus, melatonin and atenolol appear to have some adverse effects in CO intoxication on the myocardial cells.