Our recent study of the variability of blood pressure from day to day1immediately suggested other questions, such as the amount of time necessary for significant variations of blood pressure to occur and the relation of physical activity to variability of blood pressure. With the hope of throwing some light on these questions we undertook the present study. METHODS OF INVESTIGATION Since our purpose in this study was to observe the variability of blood pressure with the factors of physical activity and long intervals of time kept constant, it was decided to take readings at five minute intervals with the subject inactive throughout. One hundred men students at the University of Minnesota were selected for the study; each student sat quietly at the side of a table, while blood pressure readings were taken at five minute intervals over a period of one hour. The arm band was adjusted at the