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To determine whether orally delivered instructions can modify the intensity and direction of blood pressure and heart rate fluctuation.The blood pressure of 120 subjects, 60 hypertensive and 60 normotensive, was measured before and after oral instructions. The normotensive subjects were selected from a sample of university students and the hypertensive patients were selected at a routine medical screening. Each sample of 60 subjects was randomly divided into four groups of 15. Each subject was left seated alone in a room for 5min. The researcher then measured the subjects' blood pressure and heart rate. Following this, each group of normotensives and hypertensives was told that their blood pressure would diminish, or that it would not change or that it would increase. The control group was given no instructions. After 5 min the blood pressure and heart were measured again.In the normotensive and hypertensive groups who were told that their blood pressure would increase, systolic blood pressure increased by 4.3 and 2.5mmHg, respectively. In the groups who were told that their blood pressure would decrease, systolic pressure fell by 7.8 and 7.4 mmHg, respectively. Those who were told that no change would occur showed a systolic pressure decrease of 3.5 and 1.8 mmHg, respectively. In the control groups systolic blood pressure decreased by 5.6 and 4.2 mmHg, respectively.These results show that oral instructions are a source of variation in the assessment of blood pressure and emphasize the need for 24-h blood pressure monitoring to eliminate this type of variation.