1. Rats of the New Zealand strain with genetic hypertension (GH rats) were found to have lower plasma volume in relation to body weight than rats of the normotensive parent strain (N rats). GH rats had higher venous packed-cell volume percentage than N rats, but total erythrocyte volume was similar in the two strains of rats. Extracellular fluid volume and exchangeable sodium concentrations were lower in the GH rats. Plasma sodium concentration was not altered; plasma potassium concentration was slightly higher in the GH rats.

2. These results indicate that hypersecretion of a sodium-retaining hormone is unlikely to be a primary factor causing hypertension in the GH rats. Cardiac output has not been measured: if an increased cardiac output is a factor in the maintenance of the hypertension, it is evidently not secondary to an increased blood volume.

3. The lower exchangeable sodium concentrations and body fluid volumes found in the GH rats may be due to lower aldosterone activity secondary to a decrease in the activity of the renin-angiotensin system, or to the effects of the elevated blood pressure on the renal handling of sodium, or to both factors.

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