AchE has been histochemically demonstrated in the dLGN of different mammals. In the rat dLGN the enzyme is light microscopically localized in fibres only. This reaction is somewhat higher in the superficial than in the innermost region of the dLGN. The same distribution has been observed in other rodents like hamster, mouse, and guinea pig. Reaction products have been found electron microscopically in afferent fibres of the rat's dLGN with characteristic vesicle types and synaptic contacts to dendrites. We concluded that these fibres originate from the formatio reticularis of the brain stem. Nerve cells show a very weak reaction in the nuclear envelope and in the endoplasmatic reticulum. In the rabbit dLGN cell bodies with strong AchE-reaction are demonstrable. The AchE-activity in the laminae of the dLGN of tree shrew, cat and monkey is compared with the morphologically characterized cell types. It is discussed that X-cells in the mammalian dLGN are more influenced by AchE-containing afferent axons than other types of geniculo-cortical relay cells. Light microscopic results of AchE-localization in the visual cortex of rodents, tree shrew, and cats are described.