In anaesthetized cats reversible denervation of one kidney was performed by cooling of the left renal nerves to 3°C for 16 min. The response of the left (ipsilateral) kidney was compared with the response of the right (contralateral) kidney twice in the same animal: (1) when the right kidney was still innervated, and (2) after it had been surgically denervated. Left renal nerve cooling did not cause any changes in arterial pressure. In the left kidney, blood flow, vascular conductance, sodium and water excretions increased, and renin release decreased. Simultaneously in the contralateral kidney, no haemodynamic changes were observed, glomerular filtration was only transiently decreased, whereas sodium and water excretion significantly decreased and renin release increased. When left renal nerve cooling was repeated after surgical denervation of the right kidney, similar changes were observed in the left (ipsilateral) kidney, whereas all contralateral effects were abolished. These experiments suggest that tonically active afferent fibres from one kidney exert a reflex inhibitory action on sympathetic activity directed to the contralateral kidney controlling tubular sodium reabsorption and renin release.