1. Recordings have been obtained from single sympathetic fibres to the left kidney in anaesthetized adult spontaneously hypertensive (SH) rats and Wistar—Kyoto normotensive (WK) rats.
2. Under control conditions SH rats had a mean sympathetic activity of 3·3 ± se 0·45 Hz and WK rats 1·6 ± 0·23 Hz. During noradrenaline infusion single fibre discharge decreased graudally to low levels in both SH and WK rats. The absolute decrease in fibre activity/mmHg rise of arterial pressure was greater in SH rats than in WK rats, but the reverse was true when the discharge reduction was expressed as a percentage change.
3. Recordings have also been obtained from the left splanchnic nerve in awake SH and WK rats. Upon volume load with blood, in SH rats the sympathetic activity decreased significantly more than in WK rats.
4. Thus the sympathetic discharge to the kidneys is higher in SH rats than in WK rats. Upon volume load SH rats show an exaggerated reflex response, probably due to an augmented volume-receptor response.