Studies focused on the intake of different dietary fats have shown changes in membrane lipid composition and, as a result, alterations in membrane physical properties. These changes affect erythrocyte morphology, receptor activity and oxygen transport, among others. Here, we compare the effects of diets exclusively differing in the type of fat (olive oil rich in monounsaturates, sunflower oil rich in n-6 polyunsaturates and fish oil rich in n-3 polyunsaturates) on fatty acid composition of plasma and erythrocyte membranes and erythrocyte morphology under scanning electron microscopy in rats. Monounsaturates are highest in animals fed olive oil diets; as are linoleic and arachidonic acids in sunflower oil-fed animals and n-3 PUFAs in fish oil-fed animals. The lowest levels of arachidonic acid are found in fish oil-fed animals and so are n-3 PUFAs in sunflower oil-fed animals. Our results show that sunflower oil-fed animals present lower discocyte, the major cell shape related to tissue oxygen supply, and higher codocyte percentages than olive oil- and fish oil-fed groups. Echinocyte percentage is higher in fish oil-fed animals with respect to the other two groups. The collective data indicate that olive oil elevates monounsaturates and the number of discocytes, pointing out a possible beneficial aspect of this dietary fat.