1. Vascular reactivity was studied in Tyrode solution perfused kidneys from young (7 weeks) and mature (4–6 months) spontaneously hypertensive rats (SH rats).
2. The response to nerve stimulation was greater in the kidneys from young SH rats than in those from young control rats, both in control solution and after inhibition of the disposition of noradrenaline; both groups exhibited the same sensitivity to noradrenaline, angiotensin II and barium chloride.
3. The response to nerve stimulation was normal in kidneys from mature SH rats, but responses to noradrenaline, angiotensin II and barium chloride were greater than the control.
4. Cocaine potentiated the response to nerve stimulation more in the kidneys from mature SH rats than in those from the control rats.
5. The results suggest that renal sympathetic nerves release more noradrenaline than normal in the young SH rats, which could be an important factor in causing hypertension.
6. In the established phase of spontaneous hypertension the vascular reactivity to exogenous agonists is increased, probably as a consequence of high blood pressure; the more efficient neuronal uptake causes normalization of the response to sympathetic nerve stimulation.
