This study examined the relationship between erythrocyte sodium-stimulated lithium countertransport (Na-Li CT) and systolic blood pressure (SBP) response to exercise in 22 white normotensive men (mean age 40.6 years) completing a submaximal (85% of target heart rate) graded exercise tolerance test. These men were assessed to be at risk of hypertension on the basis of a high normal diastolic blood pressure (DBP; 80-89 mmHg), being overweight and having a rapid resting pulse rate. Na-Li CT was positively and significantly associated with the maximal SBP reached during exercise (P less than 0.01) and the lowest SBP achieved during post-exercise supine rest (P less than 0.01). The relationship with resting blood pressure was not significant. Standing DBP was the only resting blood pressure measurement significantly associated with Na-Li CT (P less than 0.05). We found an association between Na-Li CT, a membrane sodium transport system associated with blood pressure and hypertension, and the maximal SBP reached during exercise, a blood pressure measurement predictive of future hypertension, in people with normotensive resting blood pressure. This suggests that altered membrane sodium transport may occur before the establishment of hypertension, and supports the possibility that Na-Li CT may be a risk factor for this disease.