The 3 lyonnese strains of genetically hypertensive (LH), normotensive (LN) and hypotensive (LL) rats were simultaneously selected by inbreeding from one group of Sprague-Dawley rats. The breeders were chosen on the basis of two parameters: the mean of the systolic blood pressures measured between six to twelve weeks of age and the slope of the systolic blood pressure increase with age. The LH strain exhibited a mild hypertension which progressively developed during the selection and was associated with an enlarged body weight and a spontaneous increase of plasma lipids. The simultaneous selected LN strain was found to provide valuable control animals. The LL rats, which were very difficult to select, would appear useful to stress, among the differences found between LH and LN rats, those which could be linked to the blood pressure alterations.