The effect of prolonged pre-ganglionic activity induced by exposure to cold (5°C) was studied in intact and decentralized superior cervical ganglia of rats. Intact and decentralized ganglia of rats kept at room temperature served as controls. catecholamines were demonstrated histochemically using the formaldehyde-induced fluorescence method. The intensity of the fluorescence of the nerve cell bodies was estimated both visually and photometrically. Decentralization in itself had no effect on the intensity of the cell bodies for up to 16 days. Exposure to cold had no effect on the decentralized ganglia but caused a marked increase in the fluorescence intensity of some nerve cells of the intact ganglia, indicating that the increased fluorescence was mediated by the pre-ganglionic nerves. The increase lasted for the whole 16-day-length of exposure to cold. It is suggested that the observed change in the fluorescence intensity reflects an increase of the enzymes tyrosine hydroxylase and dopamine-β-hydroxylase, and thus represents a histochemical correlate of trans-synaptic induction.