A sodium-proton exchanger (NHE) is a membrane transport system taking part in intracellular sodium homeostasis. An increased NHE activity was observed in erythrocytes of primary hypertension (NTP) patients. Study group consisted of 25 NTP patients and 24 control (K). NTP patients were significantly heavier (p < 0.01) than control group (BMI 26.0 +/- 2.9 vs. 23.1 +/- 2.7, respectively). They also had significantly higher systolic, diastolic and mean arterial pressure than K (SBP 167 +/- 24.6 vs. 120 +/- 10.5; DBP 98 +/- 16.7 vs. 76 +/- 6.4; MAP 118.6 +/- 17.9 vs. 89 +/- 6.4, respectively). There was positive correlation between MAP and BMI (p < 0.02) in NTP patients. Mean NHE activity was significantly higher in NTP than in K group (9.6 +/- 3.6 vs. 8.2 +/- 2.6 mmol/L RBC/h) p = 0.05. A bimodal distribution of NHE activity in NTP group was observed. There was no correlation between NHE activity and BMI, MAP, SBP, DBP, age or positive family history of hypertension in NTP or K groups.