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Objective To determine whether genetic and non-genetic components of interindividual variation in systolic and diastolic blood pressure are constant throughout the day or are time or activity dependent.Methods We obtained 24 h ambulatory blood pressure recordings in 263 members of 68 unrelated nuclear families (i.e. parents and their offspring) representative of the Caucasian population of Rochester, MN, USA. Using the time each patient got into bed as a reference point, we identified 198 records in which this reference point was preceded by eight consecutive active hours (out of bed) and followed by four consecutive inactive hours (in bed) in which four or more blood pressure readings taken each hour were judged to be technically satisfactory. For each hourly mean for systolic and diastolic blood pressure, we estimated total interindividual variance, variance associated with concomitant variables (generation; sex within generation strata; and age, height, weight, body mass index, and abdomen-to-hip ratio within generation and sex strata), and variance associated with additive genetic effects (i.e. the chief cause of resemblance between relatives). To assess trends in each component of interindividual blood pressure variance over the 12h period, we estimated the slope of the linear regression line fit to the hourly estimates.Results For systolic blood pressure, total interindividual variance did not change significantly (slope of regression line=-0.23, P=0.717). In contrast, total interindividual variance for diastolic blood pressure was greater during active hours than inactive hours (slope of regression line=—5.53, P< 0.001). For both systolic and diastolic blood pressure, variance associated with the concomitant variables was greater during active hours than during inactive hours (for systolic blood pressure slope of regression line=—2.98, P=0.001; for diastolic blood pressure slope of regression line=-6.14, P< 0.001). Likewise, for both systolic and diastolic blood pressure, variance associated with additive genetic effects was also greater during active hours than during inactive hours (for systolic blood pressure slope of regression line=—1.65, P= 0.090; for diastolic blood pressure slope of regression line =-1.47, P=0.018).Conclusions This study demonstrates that components of interindividual variation in blood pressure are not constant, but are time or activity dependent.