In 4 groups of subjects with different clinical conditions: essential hypertension, diabetes mellitus without and with macrovascular complications, vascular atherosclerotic disease (VAD), we evaluated the plasma, erythrocyte and platelet magnesium content, the correlations between them and the relationships among all these parameters and metabolic and haemodynamic determinants. From the obtained data, it is evident that platelet magnesium content is decreased in hypertensives and in type 2 diabetics with macrovascular complications, and in subjects with VAD, while the red cell magnesium content in VAD subjects and in type 2 diabetics with macrovascular complications is increased; in type 1 diabetics we observed a decrease of the plasma magnesium level. The relationships between the plasma and cellular magnesium content, in the different clinical conditions, seem interesting.