To delineate the relative roles of genetic and environmental factors on physiological variations of blood pressure and heart rate, we performed 24-hour ambulatory blood pressure monitorings with simultaneous polygraphic sleep recordings in 28 monozygotic and 16 dizygotic healthy young male twin pairs investigated in a standardized physical and social environment. Blood pressure and heart rate were measured every 10 minutes for 24 hours. A best-fit curve based on the periodogram method was used to quantify changes in blood pressure and heart rate over the 24-hour span. Surprisingly, monozygotic twins as a group tended to have higher blood pressure values than dizygotic twins, and this difference reached the level of significance for daytime systolic blood pressure (P<.005). Although environmental influences largely controlled the mean levels and characteristics of the 24-hour systolic blood pressure variations, significant genetic effects were demonstrated for the mean levels and 24-hour patterns of diastolic blood pressure and heart rate. For both diastolic blood pressure and heart rate, the genetic effects concerned largely the same characteristics of the 24-hour profiles: the 24-hour mean, the daytime mean, the value of the evening acrophase, and the value of the major acrophase. Moreover, there was a strong genetic influence for the amplitude of the 24-hour rhythm of heart rate.