Exchangeable sodium (NaE), plasma active renin concentration and blood pressure were measured in rats with a sole remaining kidney before and after the development of hypertension induced by clipping of the single renal artery and again after unclipping. Control observations were made in sham-clipped and sham-unclipped uninephrectomized rats. Renal artery clipping caused hypertension and expansion of NaE, the latter being sustained throughout the 6 weeks during which the renal artery was constricted. Hypertension in the clipped rats was progressive over 6 weeks, whereas the expansion of NaE was not; thus the two measurements were not significantly correlated. Two rats which remained normotensive after clipping did not show expansion of NaE. Plasma active renin was elevated in comparison with the sham-clipped controls on the day after clipping, but not thereafter. Unclipping in hypertensive rats was followed by a return of NaE and blood pressure to control values. Both the sustained expansion of NaE and the transient rise in active renin probably contribute to the development of hypertension in this model, but neither alone nor together do they provide a full satisfactory explanation.