Whites with essential hypertension have high activity of cell membrane sodium-lithium (Na+-Li+) countertransport when compared with normotensives. To determine whether elevated Na+-Li+countertransport is related to the twofold higher risk of hypertension in US blacks, maximal rates of red blood cell (RBC) Na+-Li+countertransport were measured in 34 black and 21 white male college students. The race groups were similar in social and physical measurements. Mean Na+-Li+countertransport activity (mmol Li/I RBC per h) was significantly lower in blacks than in whites (0.214 ± 0.083 versus 0.295 ± 0.083, P<0.001). Countertransport activity was positively correlated with Type A behaviour among whites (r=0.45, P=0.039). Other within race correlations between Na+-Li+countertransport activity and blood pressure and cardiovascular risk factors were generally positive though not significant in whites, whereas they were small or negative in blacks. If Na+-Li+countertransport has a role in the aetiology of hypertension, it would appear to differ between blacks and whites.