SUMMARY Heart rate (HR), systolic (SBP), and diastolic (DBP) blood pressure levels were compared ID 60 young males during various resting and stressful conditions. Subjects included 29 who had and 31 who had not shown occasional casual SBP readings $ 135 mm Hg under low stress conditions. These groups were further divided into those who snowed above-average and below-average HR increases at onset of a stressful shock-avoidance task (high vs low HR reactors). High HR reactors, who were equally distributed in both casual SBP groups, also showed higher SBP than low reactors during avoidance, and higher HR and SBP during a pre-stress rest period and two other stresses, the cold pressor test and viewing an erotic dim (p's< 0.01). During less stressful conditions (relaxation in the laboratory, family doctor readings, and self-determinations at home), no HR, SBP, or DBP differences were seen between high and low HR reactors. Subjects with casual SBPs $ 135 showed higher mean SBP than those with casual SBPs< 135 under all resting and stressful conditions (p's< 0.05) and generally higher DBP as well. Highest mean SBP levels during prestress rest and later stresses were shown by subjects with both casual SBPs i> 135 and high HR reactivity to the avoidance task, and lowest by subjects with neither trait. Incidence of parental hypertension was greater among high than low HR reactors, and greatest among high reactors with casual SBPs 2: 135, suggesting that HR reactivity to stress may help predict future hypertension.