Polygraphic studies were performed in 14 fullterm newborns. The sleep behavior of the infants was correlated to the serum Mg level. Correlations could be demonstrated between serum Mg and active sleep, as well as between serum Mg and quiet sleep. With increasing serum Mg quiet sleep increased, whereas active sleep decreased. After Mg injection quiet sleep increased and active sleep decreased even more. There were also corresponding correlations between serum Mg and rapid eye movements, submental muscle tone, and gross body movements.