The influence of renal dysfunction (induced by ischaemic injury) on the development of salt hypertension was studied in rats which were exposed to 60 min of renal ischaemia when either immature or adult. Saline-drinking age-matched animals served as controls. The blood pressure, plasma urea concentration, extracellular fluid volume (ECFV) and renal mass were measured 21 and 50 days after renal ischaemia. Increments of plasma urea concentration and ECFV were considered to be indicators of renal dysfunction. Increased renal mass was used as an estimate of the degree of renal injury. In adult rats, both plasma urea concentration and ECFV were increased 3 weeks after renal ischaemia. This correlated with a pronounced increase of renal mass. A similar relation of renal mass to ECFV was still present 50 days after renal ischaemia. In rats treated when immature the increment of plasma urea concentration was much smaller and ECFV did not differ significantly from the control volumes. A mild salt hypertension developed only in those rats which were treated when immature. On the other hand, the blood pressure tended to decrease in animals treated when adult. It is concluded that mild renal dysfunction facilitates the development of salt hypertension in immature rats. This is in contrast with the reversed effects of extensive renal dysfunction in adult animals. It is suggested that the age of animals is more important for the induction of salt hypertension than the degree of renal dysfunction.