The intracellular concentrations and transmembrane fluxes of Na+and K+in erythrocytes, and plasma lipids were investigated in 30 middle-aged volunteers, before and after physical training. During the first 4 months of the study, half of the subjects (group A) were subjected to a training programme (3 h/week), while the others (group B) served as controls. At the end of the control period the group B subjects also underwent a period of training. At the end of the training, in both experimental groups, the intra-erythrocyte Na+concentration was decreased (P< 0.001); the magnitude of this decrease was related to the increase achieved in physical working capacity (r = −0.44;P< 0.05). After training the activity of the erythrocyte Na+-Li+counter-transport system was decreased (P< 0.001) in both groups, whereas Na+, K+cotransport activity was increased (P< 0.001). The training intervention did not affect erythrocyte ouabain-sensitive86Rb uptake, or the calculated rate constant for ouabain-sensitive Na+efflux. Furthermore, the plasma concentrations of high density lipoproteins (HDL)2- and HDL3-cholesterol (P< 0.001) markedly increased in both groups during the training period. However, these changes were not significantly correlated with the observed training-induced changes in erythrocyte transmembrane cationic fluxes. It is concluded that physical training decreases intra-erythrocyte Na+concentration. No significant associations between training-induced changes in plasma lipids and erythrocyte sodium balance could be demonstrated.