< jats: p> 1. From a single strain of Wistar rats two lines of normotensive rats (NR) and two lines of hypertensive rats (SHR) were developed by selective breeding.< jats: p> 2. One line of normotensive rats and one of hypertensive rats were used as recipients and the remaining two were used as donors for cross-transplantation of kidneys from hypertensive to normotensive rats and vice versa. Transplantations between the two hypertensive lines and the two normotensive ones were carried out to provide animals for control studies.< jats: p> 3. Normotensive recipients of kidneys from SHR developed hypertension whereas the recipients of kidneys from NR remained normotensive. The serum urea changes were equal in both groups.< jats: p> 4. Transplantation of normotensive kidneys reduced the blood pressure of hypertensive recipients, but transplantation of hypertensive kidneys did not alter blood pressure. The serum urea concentrations were higher in the recipients of hypertensive kidneys. These results, obtained in 3–4-month-old rats, indicate that the kidneys of adult hypertensive rats are involved in the maintenance of hypertension.< jats: p> 5. To evaluate whether the kidneys from hypertensive rats may also initiate hypertension, young normotensive recipients were given kidneys taken from the NR or SHR strain while the latter were still normotensive. Transplantation of kidneys from these SHR donors resulted in significantly higher blood pressure values and serum urea concentrations than transplantation of kidneys from NR donors.