The behaviour of Arterial Blood Pressure was evaluated, by treadmill stress testing, in a group of young subjects (15-30 years old) with one or two hypertensive siblings. The best fit of the interpolating slope function was used in interpretating the findings of haemodynamic data (A.B.P. during exercise). These data were analyzed in the same way in three control groups: normal subjects 15-30 years old; hypertensive patients aged 30-45; normal subjects aged 30-45. We compared function's coefficients and parameters in these selected groups. The results show no different response in the A.B.P during stress between the normal subject and the group, same aged, with hypertensive siblings. Significative differences in the function's coefficients, were found in the control hypertensive patients. The stress testing doesn't seem recommending in subjects at risk because of parenteral hypertension: no early alteration in A.B.P. seems to be unmasked during exercise stress. The males of the control normal subject show higher blood pressure exercise value than females. This different response is not present in the group with parenteral hypertension: a more strick familial resemblance in A.B.P. is suggested in female population.