1. Vascular reactivity and noradrenaline overflow were studied in Tyrode solution-perfused kidneys from young (6 weeks) normotensive and spontaneously hypertensive (SH) rats.
2. The vasoconstrictor response to nerve stimulation was greater in the kidneys from young SH rats than in those from young control rats.
3. Nerve stimulation evoked a greater release of noradrenaline in the kidneys from young SH rats than in those from normotensive animals.
4. The results demonstrate that the renal sympathetic nerves release more noradrenaline than normal in the young SH rats, which could be an important factor in causing hypertension.