The alteration of sodium ion transport in red blood cells was observed in SHR and patients with essential hypertension. The purpose of the present study was to determine the effects of dietary calcium intake on blood pressure and sodium ion transport of red blood cells in SHR. The SHR were fed a diet with three different levels of calcium contents as follows: 0.1% (low), 0.6% (normal) and 4.0% (high) of calcium between 6 and 20 weeks of age. At 20 weeks of age, the levels of erythrocyte sodium efflux, sodium or potassium contents in the red blood cells were measured. On the high Ca diet, SHR showed an attenuation of the increase in blood pressure. On the low Ca diet, SHR showed an enhancement of hypertension. In proportion of increasing of dietary calcium contents, SHR had a lower level of sodium content in the RBC and a higher activity of the sodium pump. However, the passive sodium permeability and sodium-potassium cotransport in SHR were similar among the three different Ca diets. It is concluded that the amounts of dietary Ca might be related to the regulation of blood pressure by changing the sodium pump of the cell membrane in SHR.