We evaluated changes in erythrocyte sodium transport systems, platelet pH, and calcium concentration induced by low and high salt intakes in a group of 50 essential hypertensive patients classified on the basis of their salt sensitivity. Patients received a standard diet with 20 mmol NaCl daily for 2 weeks supplemented in a single-blind fashion by placebo tablets the first 7 days and NaCl tablets the following 7 days. Salt sensitivity, defined as a significant rise (P <.05) in 24-hour mean blood pressure obtained by ambulatory blood pressure monitoring, was diagnosed in 22 (44%) patients. The remaining 28 (56%) were considered to have salt-resistant hypertension. In the entire group of hypertensive patients, high salt intake promoted a significant increase (P <.05) in the maximal rate of erythrocyte NA(+)-Li(+) countertransport (from 271 +/- 19 to 327 +/- 18 microM/(L cells/h) and of the Na(+)-dependent HCO3(-)-CL(-) exchanger (from 946 +/- 58 to 1237 +/- 92 microM/L cells/h) as well as in platelet pH (from 7.15+/-0 0.01 to 7.19+/-0.02 and calcium concentration (from 49+/-2 to 57 +/-2 nmol/L). Depending on salt sensitivity, high salt intake promoted opposing changes in some of the sodium transport systems studied. Salt-sensitive patients increased the maximal rate of the erythrocyte Na(+)-K(+) pump (fom 7.0 +/- 0.4 to 8.8 +/- 0.4 mmol/(L cells/h), Na(+)-K(+)-Cl(-) cotransport (from 416 +/- 37 to 612 +/- 41 micromol/(L cells/h), Na(+)-Li(+) countertransport (from 248 +/- 20 to 389 +/- 17 micromol/(L cells/h) at the end of the high salt period. Conversely, salt-resistant patients decreased the Na(+)-K(+) pump (from 8.0 +/- 0.4 to 6.9 +/- 0.3 mmol/(L cells/h) and Na(+)-K(+)-Cl(-) cotransport (from 578 +/- 53 to 481 +/- 43 micromol/(L cells/h). We conclude that modulation of erythrocyte sodium transport systems by high salt intake depends on salt sensitivity. The Na(+)-K(+) pump, Na(+)-K(+)-Cl(-) cotransport, and Na(+)-Li(+) countertransport increase in salt-sensitive patients, whereas the activity of these sodium transport systems tends to decrease in salt-resistant patients. Independent of salt sensitivity, high salt intake promotes a significant increase in the erythrocyte Na(+)-dependent HCO3(-)-Cl(-) exchanger, platelet pH, and calcium concentration in essential hypertensive patients.