The adrenergic innervation of the juxtaglomerular complex was studied in kidneys from mice, rats, guinea-pigs, rabbits, cats, dogs, pigs, monkeys, and humans using fluorescence histochemistry of neuronal noradrenaline and autoradiography of 3H-noradrenaline. The localization of the nerves was established by phase contrast optics or by perfusing the vascular system with India ink. Adrenergic nerve terminals, exhibiting a formaldehyde-induced fluorescence and having the ability to take up and accumulate 3H-noradrenaline, were easily identified when they enclosed the glomerular afferent arteriole. They continued in between and close to the macula densa and lacis cells to supply the glomerular efferent arteriole. The nerves could be seen to accompany this arteriole for a considerable distance until they branched off to the vasa recta in the juxtamedullary region and to adjacent cortical veins. This innervation pattern was found to be a constant feature except in kidneys from guinea-pigs and cats, in which post-glomerular units. The fluorescence in all adrenergic fibres supplying the juxtaglomerular complex disappeared after removal of the aortico-renal ganglion, showing that they belong to a common system of renal sympathetic nerves.