The aim of the present study was to examine the effects of awareness of hypertension on psychological factors and whether there was an association between psychological and sympathetic responses. To avoid self-selection bias 32 19-yr old white men, all with mean blood pressure of 116 mm Hg were randomized into two groups. One group was informed that the blood pressure was elevated and asked to come to a second examination while the other was invited to take part in a coronary heart disease prevention program. A cold pressor test was undertaken and the subjects completed the Karolinska Scale of Personality (KSP). Assessed by the KSP, the informed group showed lower verbal aggression (p less than 0.01), irritability (p less than 0.05), monotony avoidance (p less than 0.05) and impulsiveness (p less than 0.05), higher detachment (p less than 0.05) but no significant differences in the other subscales like anxiety, psychasthenia or factors of hostility. Information significantly increased resting blood pressure and increments in heart rate and plasma adrenaline responses to cold pressor test. Thus, both psychological and sympathetic responses were influenced by awareness of high blood pressure. There were significant correlations between less assertive behaviour and increased plasma catecholamines.