1. Phasic waves similar to those observed in the visual system of the awake and sleeping cat ("EMP" and "PGO waves" respectively) were recorded in an area surrounding brachium conjunctivum of the mesencephalic reticular formation (MRF). 2. MRF waves during paradoxical sleep (PS) (MRF PGO wave) occurred 1--2 msec after the waves recorded in the abducens nucleus (N.VI PGO waves), while MRF waves in the alert cat (MRF EMPs) occurred 35--70 msec or more after EMPs in N.VI. 3. Both MRF EMPs in the dark and MRF PGO waves preceded EMPs and PGO waves recorded in the lateral geniculate body (LGB) by 20 msec. 4. There was a time lag of about 40 msec between right and left MRF PGO waves. 5. MRF stimulation elicited short latency responses (5--9 msec) in LGB during waking and sleeping states. During PS, the same stimuli further elicited PGO waves (latency greater than or equal to 40 msec) in the same structure. 6. Stimulation of LGB elicited short latency responses (5--9msec) in MRF. 7. Unilateral MRF lesions reduced by about 50% the number of LGB PGO waves without modifying other characteristics of the PGO wave activity. 8. Following unilateral MRF lesion, LGB EMPs were observed with each eye movement in the dark. However, LGB EMPs ipsilateral to the lesion were greatly reduced in amplitude and followed the EMPs in the opposite LGB by a time lag similar to that observed between bilateral LGB PGO waves. 9. Bilateral MRF lesions completely abolished PGO waves and EMPs observed with the animal in the dark. However, EMPs observed in the light remained unmodified. 10. From the present study it was concluded that LGB and visual cortex gross potential changes related to eye movements in the alert cat have both retinal and non-retinal components. Further, the significance of the MRF area in the mechanisms of EMP and PGO wave activities was discussed.