1. Changes in sodium turnover in experimental hypertension were studied by tracer kinetic methods after injection of the long-lived radioisotope 22Na.
2. In rats treated with renal irradiation, the pre-hypertensive latent period is characterized by a modified distribution of body sodium which can be interpreted as a transfer from peripheral stores to a more centrally located pool. Before the development of hypertension, the rapidly exchangeable sodium increased by 14%.
3. The rise in blood pressure to a hypertensive level is associated with a decrease in the fractional excretion rate of sodium. During the established phase of hypertension, serial determinations of exchangeable sodium indicate rapidly fluctuating shifts of body sodium, the significance of which remains unknown.