The purpose of this study was to examine circulatory and sympatho-adrenal responsiveness in borderline hypertensives compared to established hypertensives and normotensive controls under conditions of physical and mental provocation. Measurements of plasma catecholamines or the urinary excretion of their metabolites were used as indicators of sympathetic activity and psychological variables were assessed by means of self-ratings. There were several signs of an increased neurogenic influence in borderline hypertensives. Urinary catecholamine excretion was related to body measures only in this group. During mental stress, induced by a filmed version of Stroop's colour word test, there were signs of an enhanced hypothalamic defence reaction in the borderline group, as judged by increased circulatory responses and higher plasma adrenaline levels. These signs of increased arousal could be associated with a tendency to compensate for a slightly decreased accuracy in task performance compared to controls by increasing effort. This led to a negative relationship between subjective stress and performance, present only in the borderline group. In another group of borderline hypertensives, the effects of personal control over work pace were compared to normotensives. Personal control reduced circulatory responses to mental arithmetics in controls, but had no beneficial effect in the borderline group. Also in this study, there were signs of an enhanced defence reaction in borderline hypertensives. Higher arousal levels in borderline hypertensives may, theoretically, be explained by personality differences. During an isometric handgrip test, borderline hypertensives showed a tendency towards increased alpha-adrenergic vasoconstriction compared to both established hypertensives and controls. A somewhat higher diastolic blood pressure variability, lower plasma volume and higher venous tone compared to normal also suggest increased neurogenic influences in borderline hypertension. There are similarities between the borderline hypertensive state and the circulatory and sympatho-adrenal pattern of the hypothalamic defence reaction. An enhancement of this reaction is particularly evident during mental stress, whereas somatic provocations such as an orthostatic test, a cold pressor test and physical work produce more similar responses compared to established hypertensives and controls. An increased reactivity to mental stress, especially when personal initiative is challenged, may contribute to the increased cardio-vascular morbidity of borderline hypertensives as a group.