1. The concentration of catecholamines was measured in several brain areas of the Hebrew University Sabra rat (SB rat), and in two substrains selected for their respective sensitivity (H) or immunity (N) to hypertension.
2. Hypertension was induced in SB rats by DOCA—salt, renal artery constriction and NaCl 1·7% drinking. The noradrenaline content was consistently elevated in the medulla oblongata of hypertensive animals. In other brain areas the rise in noradrenaline varied in the different types of hypertension.
3. Administration of DOCA—salt to H and N rats, while causing marked hypertension in the former, had no effect on noradrenaline in either strain.
4. Untreated, normotensive N rats had in the medulla oblongata, significantly higher concentrations of noradrenaline than did H rats.
5. Differences in brain noradrenaline may explain the inherited susceptibility or resistance to hypertension in H and N rats.