Reversible manipulation of local neurotransmission in brain areas, using controlled spatial and temporal resolution, is one of the powerful techniques used to investigate integrative aspects of brain function. We have developed a novel technique for rapidly inactivating local synaptic transmission, from outside the brain, within seconds or minutes via oxidation of target tissue using a photosensitive dye followed by photoirradiation (photo-dynamic tissue oxidation; PDTO). PDTO applied through a defined slit, sharply suppressed excitatory synaptic transmission in rat hippocampal slices and also suppressed in vivo hippocampal neurotransmission reversibly. Furthermore, we manipulated the voluntary movement of gerbils in free-field activity by application of PDTO to the striatum. Also, in freely moving kittens, the development of the visual cortex was manipulated by long-lasting application of PDTO to the eye. Thus, PDTO enables external manipulation of in vivo or in vitro neurotransmission in various clearly defined regions on the submillimeter scale. Suppression of neurotransmission occurred only within the photo-oxidized area which can be histochemically visualized.