We measured the cardiovascular and sympathetic nervous responses to mental stress in subjects with hyperthyroidism. Ten hyperthyroid subjects and 10 age- and sex-matched normal subjects performed mental arithmetic. At rest, the heart rate was higher in hyperthyroid subjects than in normal subjects, but systolic blood pressure, plasma norepinephrine, and epinephrine concentrations did not differ between the two groups. Systolic blood pressure and heart rate during stress, and the changes in blood pressure and in plasma epinephrine concentration from rest to stress, were higher in hyperthyroid subjects than in normal subjects. Therefore, cardiovascular and adrenal responses to mental stress were abnormally high in subjects with hyperthyroidism.