The basic functions of amineoxidases, such as the modulation of cell growth and differentiation have been described previously. In the present work we summarize some new findings of our groups on the physiological role of the copper amine oxidases. At the molecular level, it was reported that free amino groups of polylysine with different molecular weights (up to 50 KDa) and configurations (D and L) can be oxidized by bovine serum amine oxidase (SAO) with production of fluorimetric detectable H O₂. The free amino groups on some native proteins containing lysine, such as lysozyme and ribonuclease A can also be recognized and oxidized by SAO, suggesting its possible involvement in the process of protein post-translational modification. At the cellular level, a new function of SAO as a modulator of electric properties of ionic channels in neuronal cells was observed. SAO can increase significantly the current of the K⁺ channels in N1E-115 neuronal cells in a time dependent manner, reaching its maximal effect after 5 min of incubation. At the organ level, it was found that SAO possesses an important protective effect on the cardiac functions. SAO has an antiarrhythmic effect on the isolated rat heart subjected to regional ischemia at reperfusion. Untreated hearts exhibit 100% incidence of the irreversible ventricular fibrillation (IVF), while pretreatment with 0.5 μM SAO completely protected the hearts (0% IVF) after 10 min of reperfusion. Furthermore, it was found for SAO an important oxygen free radical scavenging activity (in vitro) and cardioprotection on isolated heart (ex vivo) against deleterious effects of electrolysis induced reactive oxygen intermediates.