The kinetic parameters Vmax and Km of erythrocyte sodium-lithium countertransport (Na-Li CT), and the lithium cell to plasma ratio (Li C:P) measured 24 h after a 1-g oral dose of Li2CO3, were determined in 14 normotensive (NT) and 14 untreated mild hypertensive (HT) males matched for age and weight. Li C:P and Na-Li CT were strongly correlated (r = -0.73), p less than 0.001), confirming the validity of Li C:P as an in vivo index of Na-Li CT. No differences in Li C:P (NT: 0.27 +/- 0.07; HT: 0.23 +/- 0.06), Na-Li CT (NT: 0.39 +/- 0.10 mmol/L/h; HT: 0.43 +/- 0.10 mmol/L/h), Vmax (NT: 0.58 +/- 0.16 mmol/L/h; HT: 0.68 +/- 0.20 mmol/L/h), or Km (NT: 1.5 +/- 0.4 mmol/L; HT: 1.6 +/- 0.9 mmol/L) were found between NT and HT. Our data do not support the marker concept of erythrocyte Na-Li CT in human hypertension.