Basal and casual blood pressure measurements have been obtained under standardized conditions of study in 53 adult pairs of twins who were determined to be in good general health on the basis of health histories and medical examinations. These data have afforded an analysis of: differences between the left and right arms, and differences between casual and basal blood pressure measurements in individuals, and within and between pairs of twins. In both the casual and basal studies it can be seen from the critical MZ:DZ ratios that variability in the measured levels of the blood pressure (in these adult subjects in good general health) is predominantly under environmental influences. However, different comparisons of the monozygotic twin intrapair casual and basal variances indicate that there may be genetic as well as sex influences which are not effectively defined by single blood pressure measurements, at least in the normal range of this variable. It was found that: (1) Although the average of the blood pressures is unaffected by the arm employed, differences between pressures on the left arm and those on the right arm are an important consideration when comparing measurements taken on the same individual, or between the two members of a pair of twins. (This would also be an important consideration in the study of siblings and families.) (2) The conditions of study have a greater effect upon the interpretation of blood pressure data than might be implied by the relatively small differences in the average of the blood pressures taken under varying conditions of study. (3) Measurement of genetic, environmental, and sex influences upon blood pressure, as well as the association of blood pressure with such variables as weight and ponderal index, depend upon the conditions of study. (4) There are genetic and sex-influenced factors related to cardiac function which are not effectively described by single measurements of the level of the blood pressure. The analyses of these data on twins pose the question whether measured levels of the blood pressure constitute the most critical manifestation of the genetic and sexinfluenced variables to be investigated. The lability of the blood pressure, its reactivity to specific stimuli, the relationship of systolic to diastolic pressure, and, possibly, the correlation of blood pressure with other attributes may be the most promising avenues for further genetic investigations of this clinically important measurement.