Studied was the central cardiac rhythm control with the use of the statistical indices of cardiac cycle length (variation range, mode, mode amplitude) on the background of altered activity of Earth's magnetic field as judged by the D, H and Z constituents. Subjects were 36 essentially healthy humans of two age groups: 19-23-yr. old (group-I) and 51-53-yr. old (group-II). During magnetic storms the strain index increased in 44.4% and 66.6% of the members of groups I and II, respectively. In group I, this parameter rose by 60 +/- 5.8% and in group II, by 85.6 +/- 8.8%. Changes in the indices were more pronounced in group II. Males are more sensitive to alterations in the magnetic field strain. There were no subjective sensations despite the registered magnetobiological reactions. Results of the investigation showed that magnetic storms are associated with higher activity of the sympathoadrenal and hypothalamus--pituitary body--adrenals systems which is indicative of recruitment of defense mechanisms and functional strain.