Mg++ blood concentrations were determined before and after a standard exercise of 80 watts and 8 minutes from 306, mainly male probands, which were subdivided into three groups: Group 1 consisted of probands with Mg++ concentrations of 0.45 mM/l and lower. Group 2 of those with Mg++ concentrations between 0.45 mM/l and 0.5 mM/l and Group 3 of probands with Mg values above 0.5 mM Mg/1. It turned out that in groups with lower Mg++ concentrations there was also a higher blood glucose, higher lactate, lower pH and lower buffer capacity, sometimes even before exercise. If hypomagnesemia is to be defined by concomitant worsening of the metabolism, the so called hypomagnesiemic threshold should be increased from 0.45 mM/l to 0.5 mM/l, nearly a third of the physiological range.