The effects of stress on leukocyte membrane sodium efflux rate constant and plasma norepinephrine levels were studied before and during cold pressor test in normotensive subjects with and without a family history of hypertension. After 20 minutes of supine rest, no significant differences in total, ouabain-resistant or ouabain-sensitive sodium efflux rate constants were apparent between the two groups. In normotensive subjects with no family history, there was no significant change in any efflux rate constant during cold pressor test, although there was a highly significant negative correlation between change in total efflux rate constant and change in norepinephrine levels (r = −0.82,p< 0.01,n= 12). During cold pressor test in subjects with a family history of hypertension, there was a significant rise in the ouabain-resistant efflux rate constant (1.5 ± 0.1 vs 1.0 ± 0.1 hr;p< 0.01,n= 10); this level was also significantly higher than that in control subjects (p< 0.002). In this group, the ouabain-sensitive efflux rate constant fell slightly but not significantly (1.8 ± 0.2 vs 2.1 ± 0.2 hr-' a = 10). These results suggest that stress in the form of a cold stimulus produces qualitative differences in leukocyte cation transport in normotensive offspring of hypertensive patients as compared with subjects without such a family history.