The paper deals with the results of electrocardiographic investigations of relatively resting 59 cosmonauts at the age of 30 to 52 during long-term space missions. Heart rate (HR), T-wave amplitude and an electric systole interval were analyzed. During their missions, the cosmonauts consistently combined HR rises with aggravated repolarization manifested by a relative elongation of the phase and T-depression. All alterations were statistically significant (p < 0.05); however, no pathology was detected in the myocardial bioelectrical activity. With this dynamics, values of the parameters under study did not, as a rule, leave the boundaries of the physiological norm.