SUMMARY Comparative studies of plasma norepinephrine in patients with essential hypertension and in nonnotensive controls have consistently reported higher mean resting levels of norepinephrine in the hypertensive groups, but the hypertenslve-nonnotenslve differences have often been small and, in about three-fifths of the studies, not statistically significant. The author reviewed the medical literature to test the hypothesis that, during stress, hypertenslve-nonnotenslve differences in norepinephrine become more apparent. Among 24 studies involving orthostatic stress, the increment in norepinephrine with standing was similar for hypertensives and normotensives (239 vs 230 pg/ml). In contrast, among eight studies involving exercise, the increment in norepinephrine was significantly greater in hypertensives (834 vs 450 pg/ml). For both standing and isotonic exercise, absolute changes in norepinephrine with stress correlated with basal norepinephrine across the hypertensive but not the nonnotensive groups. These results are consistent with the existence within the hypertensive population of a subgroup of patients with elevated norepinephrine levels at rest and excessive sympathetic responsiveness to stress. However, the available literature is deddedly lacking in studies about other types of stress besides standing and exercise.