1. The acid–base status of young spontaneously hypertensive rats (SHR) was compared with that of Wistar–Kyoto rats (WKY) in the steady state, after acid loading and after blood pressure had been maintained at normal levels from weaning. Whole blood ionized calcium was measured simultaneously.

2. In the prehypertensive stage (4 weeks of age), plasma bicarbonate was significantly lower in SHR than in WKY, while blood pH did not differ significantly.

3. After 6 weeks of age, blood pH and plasma bicarbonate were significantly lower in both anaesthetized and conscious SHR than in corresponding WKY. After 7 days administration of NH4Cl in the drinking fluid, both parameters decreased significantly in both strains and the difference in pH remained constant (0.05 pH unit, P < 0.01).

4. In none of the groups investigated did non-pH-adjusted ionized calcium differ significantly between the SHR and WKY.

5. Prevention of the development of hypertension in SHR by hydralazine treatment from weaning did not increase pH or bicarbonate compared with untreated SHR, indicating that the metabolic acidosis in the SHR was not a consequence of raised blood pressure.

6. Disturbance in acid–base balance may be involved in the pathogenesis of raised blood pressure in this animal model of genetic hypertension.

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