Introduction:Several authors have described increased Na+— H+exchanger activity in essential hypertension, and an increase in activity of this transport system has also been postulated in situations of hyperinsulinism, such as obesity and essential hypertensionMethods:We measured Na+— H+exchanger activity in a group of 37 subjects with essential hypertension (18 obese, 19 non-obese), in a group of nine normotensive obese subjects and in a control group of 16 healthy volunteers. Plasma insulin and glucose values during an oral glucose tolerance test were evaluated, together with other variables such as plasma aldosterone, plasma renin activity and plasma potassiumResults:Na+—H+exchanger system activity did not appear to be abnormally raised in the hypertensive subjects, but was significantly increased in the normotensive obese group. Upon dividing the hypertensive subjects into two subgroups on the basis of body mass index, it was noted that, whereas the non-obese hypertensives showed Na+—H+exchanger activity patterns similar to those in controls, the obese hypertensive subjects exhibited increased activity of the transport system. Na+—H+activity correlates with body mass index and shows a significant inverse correlation with plasma potassium. No correlations were found between Na+— H+exchanger activity and the sum of plasma insulin values during the oral glucose tolerance testConclusion:Na+— H+exchanger overactivity appears to be characteristic in overweight subjects, but would not appear to be a specific feature of essential hypertension. The increased Na+— H+exchanger activity observed in obese subjects may be postulated to be related to the hypermineralocorticoidism characteristic of this condition