Sodium efflux from normal red cells was measured as a function of pressure to test whether abnormal sodium transport in hypertension is a direct consequence of the increased arterial pressure. Red cells were loaded with 22Na and sodium efflux was measured at 37 degrees C while the samples were in a bomb at constant pressures of 200 mmHg or 517 mmHg. Control samples were incubated concurrently at atmospheric pressure and the same temperature. The effect of preincubation of blood at 200 mmHg for 3.5 h on sodium efflux was also measured. 22Na efflux and first order efflux rate constants were similar in high and normal pressure samples in each case. These findings suggest that acute changes in pressure have no effect on erythrocyte sodium efflux, which in turn implies that abnormal membrane transport in hypertension is not a consequence of the raised arterial pressure.