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The effect of blood pressure and heart rate reactivity on respective variability in everyday life conditions was studied in a group of middle-aged, normotensive men (n=22, mean age 39.3 years, range 35-45 years). Continuous intra-arterial tape recording was used to measure 24-h blood pressure and heart rate and the subjects completed a 24-h behavioural diary. The variability of blood pressure and heart rate was analysed using 30-s averages and cumulative distribution curves. The overall blood pressure and heart rate variability was calculated as the difference between the 90 and 10% levels of the cumulative distribution curves. Reactivity caused by different daily activities was calculated as the change from baseline level, defined as the 10% level determined from the cumulative distribution curve of waking hours. All regular activities were monitored. Large interindividual differences were found in variability and reactivity. The mean reactivity to different activities varied from 3.3 to 44.7 mmHg for systolic blood pressure, from 1.8 to 16.3 mmHg for diastolic blood pressure and from 2.0 to 46.0beats/min for heart rate. The mean contribution of reactivity to variability varied between 21 and 74% for systolic blood pressure, from 19 to 58% for diastolic blood pressure and from 20 to 82% for heart rate. As expected, blood pressure and heart rate levels were significantly higher at work than at home. We conclude that the reactivity caused by daily activities has a pronounced influence on blood pressure level and variability during waking hours.