The effect of chronic saline loading for 3 weeks has been studied on the fractional distribution of cardiac output and intrarenal circulation in the rat. In response to the saline loading extracellular volume was slightly expanded and cardiac output moderately increased, the extent of which seemed to correlate with the fluid intake. Fractional organ flow distribution remained unaffected. Long-term saline loading resulted in a redistribution of intrarenal circulation characterized by an enhanced medullary blood flow, either isolated, or associated with an eventual dispropartional increase of cortical flow, while the renal fraction remained unaltered. On the basis of the results it is concluded that during long-term saline loading the augmentation of medullary circulation is a specific phenomenon characteristic of the kidney and is independent of the changes in cardiac output or other systemic haemodynamic effects. The isolated increase of medullary circulation is claimed to have a volume-regulatory function.