Twenty-four adolescent boys with a positive and 40 boys with a negative parental history of hypertension (PH+ and PH-, respectively) were studied at rest and during exercise on a bicycle ergometer at work loads ranging from 25 to 150 W. Each group was divided into fit and unfit categories on the basis of oxygen consumption at a heart rate (HR) of 150 beats/min. Blood pressure (BP) at rest averaged 114.2/68.8 mmHg in the PH+ group and 110.9/70.3 mmHg in the PH- group. Age and body weight did not differ between subgroups, although resting HR was lower in fit than in unfit subjects (P less than 0.01). Oxygen consumption increased with exercise but did not differ between groups at any work level. However, systolic BP was significantly higher in the PH+ group than in the PH- group at both 100 W (163.1 +/- 12.3 versus 156.7 +/- 12.2 mmHg) and 150 W (174.3 +/- 12.5 versus 166.6 +/- 10.4 mmHg, P less than 0.05). These results suggest that the exaggerated BP responses to exercise, characteristic of hypertensive patients, may be present in normotensive adolescents with an increased risk of developing the disorder, and may reflect pathophysiological changes that precede sustained BP elevation.