It was discovered in 1966 that the senegalese baboon (Papio papio) exhibits a photosensitive epilepsy. This finding has led, among other work, to the neurophysiological study of this epilepsy. Although in some characteristics the baboon's photosensitive epilepsy differs from that of man, it can be considered that this animal presents a real model of essential epilepsy, for the study of the human disease. 2. In the baboon, the EEG disturbances triggered by intermittent light stimulation at 25 Hz appear first at the level of the frontal cortex (area 6). At this level, recordings of single unit discharges show an activation of cortical neurones similar to that observed in human patients with focal epileptic lesions ; at the occipital level, the only modification observed is a change in the resting membrane potentials, in the direction of disinhibition. 3. The analysis of cortical visual evoked responses demonstrated the presence of short latency visual afferents at the frontal cortex level, as well as a high level of hyperexcitability for the visual modality. The most photosensitive animals can be distinguished by a more marked frontal hyperexcitability and by slight differences in the form of both the occipital evoked responses (decrease in amplitude of the early part of the response, frequent absence of wave IV) and the frontal ones (higher amplitude of the later part of the responses). In some of the animals, whether they were photosensitive or not, we found high amplitude frontal visual evoked responses resembling spikes and waves. 4. Certain observations in both man and the photosensitive baboon suggested the possible involvement of periocular somatic afferents in the triggering of paroxysmal manifestations. The study of these cortical projections in the baboon showed that they possess certain specific characteristics which distinguish them from the other somatic projections (short latency, large frontal spread and ipsilateral responses of higher amplitude than contralateral). It seems, however, that if they play a role in the epileptic manifestations, these periocular projections are not required to trigger this behaviour. 5. The baboon frontal lobes seem therefore to be an area receiving multimodal projections, possessing a particular sensitivity to visual afferents, and functionally equivalent to a zone of focal epilepsy as might be met with in man or animals. 6. The results are discussed in the light of observations made on the same or other species, or on other types of epilepsy. In particular, the totality of the visual afferents arriving at the occipital level appears to be necessary to trigger epileptic manifestations. Finally, these results present several arguments in favour of the cortical theory of generalised epilepsies, as well as substantiating the value of the baboon as an animal model for photosensitive epilepsy in man.