A retrograde axonal transport of nerve growth factor (NGF) from the adrenergic nerve terminals in the mouse iris to the cell bodies of postganglionic sympathetic neurones in the superior cervical ganglion has been demonstrated. After injection of iodinated nerve growth factor (125I-NGF) into the anterior eye-chamber there was a relatively rapid accumulation of radioactivity in the superior cervical ganglia on both injected and non-injected sides, as was the case after subcutaneous injection. However, 4 h after intraocular injection a preferential accumulation of radioactivity became apparent in the superior cervical ganglion on the injected side, and this difference between the ganglia on injected and non-injected sides gradually increased to a maximum at 16 h. Transection of the postganglionic adrenergic fibres as well as the prior intraocular injection of colchicine abolished the preferential accumulation of 125I-NGF in the superior cervical ganglion of the injected side, whereas the destruction of adrenergic nerve terminals by 6-hydroxydopamine did not impair the preferential accumulation. It is concluded that the retrograde axonal transport of NGF, which was estimated to take place at a rate of about 2.5 mm/h, depends on a colchicine-sensitive mechanism as does the orthograde rapid axonal transport. However, the uptake of NGF may not only take place from the nerve terminals but also from the preterminal parts, as has been shown in other studies with horseradish peroxidase. Autoradiographic studies strongly supported the existence of a retrograde transport by showing a clear localization of radioactivity in a small number of neurones in the superior cervical ganglion on the injected side, whereas on the non-injected side there was only a diffuse distribution of radioactivity throughout the ganglion.