1. In this study we sought to test the possibility that an olive-oil-rich diet might influence the membrane lipid composition and cation transport mechanisms in erythrocytes. 2. To this end, 11 normotensive normolipidaemic healthy volunteers were given a standard relatively low fat diet (28% of total energy) for 3 weeks, followed by a high fat (38% of total energy), olive-oil (100 g/day)-enriched diet for the 3 subsequent weeks, in a metabolic ward. The two diets contained the same amount of cholesterol (400 mg), and the polyunsaturated fatty acid/saturated fatty acid ratio was similar and comparatively low (0.35 on the standard vs 0.36 on the olive oil diet). 3. At the end of the study the lipid composition of the erythrocyte membranes showed the following changes: the oleic acid content increased by 15.7% and the amount of saturated fatty acid decreased by 3.2%. The polyunsaturated fatty acid content tended to decrease, while the polyunsaturated fatty acid/saturated fatty acid ratio did not change significantly. The cholesterol/phospholipid molar ratio increased significantly by 7.8%, still remaining within the reference range. 4. At the same time, the mean values of the maximum rates of the cation fluxes mediated by the Na+ -K+ pump, and by Na+ -K+ co-transport rose significantly, while the maximal rate of Na+ -L+ countertransport showed a non-significant trend towards lower values. 5. Urinary excretion of Na+ and K+, body weight and blood pressure did not change significantly.