Two groups of individuals, 26 normotensive normolipemic and 37 normotensive hyperlipemic, all without family history of hypertension have been selected in attempt to demonstrate whether Li-Na countertransport of erythrocytes is influenced by plasma and membrane lipid composition. The maximal rate of Li-Na countertransport was elevated in hyperlipemics (0.344 +/- 0.168 vs 0.220 +/- 0.074 mmol/l erythrocytes/h). This difference is highly significant. Hyperlipemics had different composition of membrane lipids than normals. The most important variations were: increase of palmitic, palmitoleic and total saturated fatty acids (SFA) as well as increase of cholesterol/phospholipids ratio (C/PL); in contrast, hyperlipemics had a reduced amount of linoleic acid and total unsaturated fatty acids (UFA) as well as total polyunsaturated fatty acids (PUFA). Consequently, UFA/SFA and PUFA/SFA ratios were lower than in normals. Li-Na countertransport was negatively correlated with the amount of PUFA (P less than 0.02), whereas it was positively correlated with the following parameters: oleic/linoleic ratio (p less than 0.02), monounsaturated fatty acids/polyunsaturated fatty acids ratio (p less than 0.03) as well as with the SFA + monounsaturated fatty acid/PUFA ratio (p less than 0.03). These findings suggest that the V max of Li-Na countertransport in erythrocytes is influenced by the lipid composition of the membrane.