The roll-over test has been reported to predict pregnancy-induced hypertension. The arm used to record blood pressure has never been specified. Since the level of the arm with reference to the heart is of great importance in the interpretation of blood pressure, we recorded the roll-over response (blood pressure change from left lateral to supine position) first in the right and then in the left arm of 9 men and 8 nonpregnant women. In addition, blood pressure was measured simultaneously in both arms in 10 pregnant primigravidas. In the men and nonpregnant women the mean diastolic blood pressure after rolling over rose when the right arm was used and fell when the left arm was used. The difference in the roll-over response (right versus left arm) was accounted for by the hydrostatic effect produced by the vertical distance between the corresponding brachial arteries with the subject in the left lateral recumbent position. In the pregnant subjects as well, there was a difference between the roll-over response in the right versus left arm, accounted for by hydrostatic factors. However, the increase in blood pressure was greater for both arms, suggesting a real change in central blood pressure.