SUMMARY We have investigated the changes in plasma norepinephrine and blood pressure and heart rate during a range of physical activities in eight hypertensive subjects in order to determine whether changes in plasma norepinephrine reflect changes in sympathetic activity. Blood pressure was recorded over 24 hours from an intra-arterial cannula. Plasma norepinephrine, measured by a sensitive radioenzymatic method, increased progressively with increasing levels of physical activity. In each subject a statistically significant linear relationship was observed between the logarithm of plasma norepinephrine and systolic blood pressure. Analysis of variance showed that 66% of the variance of plasma norepinephrine was associated with changes in blood pressure and heart rate. These observations support the hypothesis that plasma norepinephrine reflects short-term changes in sympathetic activity. Use of the quantitative relationship described, in conjunction with measurements of norepinephrine metabolism, may help to determine the significance of increased levels of plasma norepinephrine observed in some hypertensive patients.