Recent works have shown the importance of blood pressure (BP) response to exercise or to daily activity as determinants of hypertensive cardiac hypertrophy. We have tested the relationship between BP and left ventricular (LV) mass in 23 normotensive young adults (mean age 25 +/- 5) with normotensive parents. Blood pressure was measured at rest, at the end of maximal exercise test, and during 24-hour ambulatory monitoring during daily routine. Left ventricular mass was assessed with M-mode echocardiography. We found a closer relationship between LV mass with BP when measuring during activity (average day time BP and end exercise BP) than with rest BP (rest BP and average nighttime BP). Multiple regression analysis indicates an independent association of LV mass with both average day time and end exercise systolic blood pressure. Thus, we conclude that these two ways of assessing "activity" BP may be more complementary than opposing. Ambulatory BP monitoring appreciates the reactions to daily stresses and the exercise test measures the response to physical activity.