1. Exchangeable body sodium was measured in deoxycorticosterone acetate (DOCA) -salt-treated rats by whole body γ-counting after equilibration with a 22Na isotope. 2. The blood pressure of DOCA-salt-treated rats is positively correlated with their exchangeable body sodium. 3. After pithing, the heart rate is significantly lower in DOCA-salt-treated than in control rats: in conscious DOCA-salt-treated rats the heart rate is also lower than in control rats but the difference was not statistically significant. 4. Cardiac and vascular responses to sympathetic stimulation are positively correlated with exchangeable body sodium in pithed rats, but there is no correlation between either cardiac or vascular responses to noradrenaline injections and exchangeable sodium. 5. Pressor responses to sympathetic stimulation in pithed DOCA-salt-treated rats are potentiated less by cocaine than are responses of untreated control rats. 6. It is suggested that sodium retention in mildly hypertensive rats specifically enhances responses to sympathetic stimulation by increasing the availability of the sympathetic transmitter.