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1. In rabbits anaesthetized with Dial ACh has been collected from the surface of the cerebral cortex during stimulation of the visual pathways.

2. The spontaneous release of ACh from the visual and non-visual areas of the cortex was found to be similar.

3. Stimulation of the retinae by diffuse light produced a large increase in ACh release from the primary visual receiving areas (4·3 times the spontaneous release) and a smaller increase (1·9 times the spontaneous release) from other parts of the cortex.

4. Direct unilateral electrical stimulation of the lateral geniculate body evoked a large increase in ACh release (3·4 times the spontaneous release) from the ipsilateral visual cortex and a smaller increase (1·7 times the spontaneous release) from the contralateral visual area and other regions of the cerebral cortex. The evoked increase from the contralateral cortex was not mediated by transcallosal pathways.

5. The increase in ACh release evoked from the visual cortex by stimulation of the ipsilateral lateral geniculate body was dependent on the frequency of stimulation. The evoked release was smallest at low stimulus frequencies and increased to a maximum at 20 stimuli/sec.

The evoked ACh release from other areas of the cortex was independent of the frequency at which the lateral geniculate body was stimulated.

6. The possible central nervous pathways associated with the spontaneous release of ACh and the release evoked by stimulation of the eyes by light and by direct stimulation of the lateral geniculate body are discussed.

7. It is concluded that two ascending cholinergic systems may be involved; the non-specific reticulo-cortical pathways responsible for the e.e.g arousal response, and the more specific thalamo-cortical pathways associated with augmenting and repetitive after-discharge responses. The first system is thought to be concerned with the small but widespread increase in ACh release from the cortex following stimulation of the visual pathway while the second system could give rise to the larger increases evoked from the primary receiving areas of cortex. The spontaneous release of ACh from the surface of the brain may be the result of contributions from both systems.