1. There was a significant decrease in the activity of tryosine hydroxylase, dopamine-β-hydroxylase and phenylethanolamine-N-methyltransferase in young spontaneously hypertensive rats (SHR) at the age of 2 and 4 weeks, compared with the activity in adrenals of the Wistar-Kyoto rat substrain. Tyrosine hydroxylase activity was not different in normotensive and hypertensive animals at 8 weeks of age but was increased in adult SHR.

2. Both dopamine-β-hydroxylase and phenylethanolamine-N-methyltransferase activities were still decreased at the age of 8 weeks in SHR, but were not significantly different from the controls at the age of 14 weeks. Blood pressures in SHR were slightly but significantly higher at 4 weeks of age and steadily rose during maturation.

3. In DOCA-saline hypertensive rats, tyrosine hydroxylase activity in adrenals was increased after only 1 week of treatment and remained increased after 2 and 4 weeks of treatment. Adrenal adrenaline was increased after 4 weeks of treatment, whereas dopamine-β-hydroxylase and phenylethanolamine-N-methyltransferase activities and noradrenaline levels were unchanged.

4. Our findings of increased tyrosine hydroxylase in adrenal glands of adult genetically hypertensive and chronic experimentally hypertensive rats indicate a sympathoadrenal activation during the established phase of the hypertension in both models. Whereas in DOCA-saline hypertensive rats catecholamine synthesis in adrenals parallels the development of high blood pressure, in genetically hypertensive rats the activity of the catecholamine-synthesizing enzymes is decreased early in development. These results suggest the existence of different mechanisms regulating the participation of adrenal catecholamines in both models of experimental hypertension.

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