The response of blood pressure, heart rate and plasma catecholamines to a mental arithmetic and a cold pressor test was studied in 70 patients with mild essential hypertension and in 41 age- and sex-matched normotensives. Each group consisted of three prospectively stratified age classes: 20-29, 30-39 and 40-55 years. During mental arithmetic, hypertensives showed only a higher increment of systolic blood pressure (+17-19%) than normotensives (+12-15%). Plasma noradrenaline in the youngest normotensives (20-29 years) showed a small but significant decrease (-0.20 +/- 0.07 nmol/l) whereas the youngest hypertensives showed a small but significant increase of plasma noradrenaline (+0.14 +/- 0.04 nmol/l). The difference between both groups was highly significant (P less than 0.001). In the two older age classes there was no difference in plasma noradrenaline response between normo- and hypertensives. During the cold pressor test both the cardiovascular and plasma noradrenaline response were of the same magnitude in normo- and hypertensives. These data reinforce the concept that the increased sympathetic reactivity to mental stress in hypertensives may be restricted to the younger age.