Evidence indicates that an alteration in kidney function has a causal role in the pathogenesis of hypertension in the Milan hypertensive strain (MHS) rat. At the prehypertensive stage, these animals have increased glomerular filtration rate and 24-hour urinary output, whereas plasma renin activity and urinary kallikrein are lower. After transplantation, the MHS kidney increases the blood pressure of a normotensive recipient. Micropuncture experiments, where single nephron filtration rate, tubuloglomerular feedback, proximal tubular reabsorption, micro-pressures in tubuli, and interstitium and interstitial oncotic pressure were measured, suggest that the intrinsic ability of MHS proximal tubular epithelium to reabsorb solute and water is greater in prehypertensive MHS rats than in Milan normotensive strain (MNS) rats. Also rheogenic Na transport across the brush-border vesicles isolated from proximal tubular cells is faster. When erythrocytes and proximal tubular cells of MHS rats are compared to those of MNS rats, the former have smaller volume and Na content, whereas the Na transport is faster and the Ca ATPase at Vmax is lower. This indicates that the genetic cellular abnormality responsible for the renal functional abnormality and hypertension is also present in erythrocytes. Moreover, MHS erythrocyte abnormalities are genetically determined within the stem cells and are genetically associated with hypertension. Because a correlation was also found in human hypertension between erythrocyte Na transport abnormality and renal function, it is proposed that the erythrocyte may be used for studying the genetic molecular mechanisms of hypertension.