Changes in plasma endogenous ouabain-like substance (EOLS) and plasma noradrenaline, along with changes in blood pressure (BP), heart rate, and blood lactate concentration, were investigated in hypertensive individuals during strenuous exercise testing. Thirteen (4 men and 9 women) middle-aged (34-63 years, mean 50 +/- 2 years) patients with mild essential hypertension underwent graded multistage submaximal exercise testing on a cycle ergometer. The workload in each exercise test was increased depending on the individual's physical fitness until they reached 75-80% of the predicted age-adjusted maximal heart rate prescribed by the American College of Sports Medicine. Systolic (S) BP rose by 67 +/- 9 mm Hg (P < 0.001), mean (M) BP by 28 +/- 3 mm Hg (P < 0.001), diastolic (D) BP by 9 +/- 2 mm Hg (P < 0.005) and heart rate by 79 +/- 6 beats/min (P < 0.001) after submaximal graded exercise. The blood lactate concentration and plasma noradrenaline increased significantly (+3.40 +/- 0.34 mmol/l and +895 +/- 94 pg/ml respectively, P < 0.001). Although the change in EOLS was not significant, it showed a strong positive correlation with the change in plasma noradrenaline (R = 0.760, P < 0.001). These results suggest that EOLS may participate in modifying sympathetic vasoconstriction during submaximal graded exercise.