Magnesium , a miracle mineral has a healing effect on a wide range of diseases. It is essential for many enzyme reactions especially in regard to cellular energy production for the health of brain and nervous system and also for healthy teeth and bones. It is the most important mineral for maintaining proper electrical balance and facilitating smooth metabolism in cells and has a membrane stabilizing effect. AIMS AND OBJECTIVES :1.To determine the level of serum magnesium in non–pregnant women of child–bearing age .2. To determine the level of serum magnesium in toxaemias of pregnancy like pre–eclampsia and eclampsia.3.To compare serum magnesium levels of non–pregnant women and normal pregnant women in their third trimester with the serum magnesium levels of patients with pre–eclampsia and eclampsia. 4.To compare serum magnesium levels of pre–eclamptics and eclamptics. MATERIAL AND METHODS : Study was carried out in the Deptt. Of Obstetrics and Gynaecology, Patna Medical College and Hospital, Patna during September 2006 to September 2008. Estimated by colorimetric method using “Titan Yellow” Result & Discussion : The co–existence of toxaemic conditions of pregnancy, further reduces the serum magnesium level in comparison to non–pregnant and uncomplicated pregnancies. Further serum magnesium levels were significantly lower in patients with eclampsia when compared with the serum magnesium levels of pre–eclamptic patients