The possible association of plasma lipids (triglycerides and cholesterol) with erythrocyte Na+ content (Na+j) and/or with alterations in red cell Na+ and K+ (Rb+) transport was studied in a population of F2 hybrids obtained by crossing hypertensive Prague hereditary hypertriglyceridaemic (HTG) rats with normotensive Lewis rats. The obtained data indicated a strong cosegregation (p<0.001) of plasma triglycerides with erythrocyte Na+ content. This was the cause for the close correlation of plasma triglycerides with the Ma+-K+ pump activity (measured as ouabain-sensitive Na+ extrusion). On the contrary, there was only marginal association (p<0.05) of erythrocyte Na+ content with plasma cholesterol which was significantly (p<0.01) related to bumetanide-sensitive Rb+ uptake mediated by the Ma+-K+ cotransport system. Na+ leak (bumetanide-resistant net Na+ uptake) correlated positively with blood pressure in female but not in male F2 rats. The close association between plasma triglycerides and erythrocyte Na+ content suggests that ion transport alterations might contribute to mechanisms responsible for the cosegregation of blood pressure with plasma triglycerides in HTG x Lewis F2 hybrids.