1. The slow surface-negative wave produced in the rabbit visual cortex by single afferent or direct stimulation, which is regarded as a reflection of recurrent inhibition of cortical neurons, is depressed when tested against a background of tonic stimulation of the mesencephalic reticular formation (RF). 2. Analogous depression is observed after short (a series of 2 or 3 stimuli at an interval of 3–5 msec) stimulation of the RF. This depression develops very quickly and reaches a maximum when the interval between conditioning and test stimuli is about 5 msec. 3. Similar short RF stimulation increases activity of most cortical neurons. This burst of activity has a latent period of about 20–35 msec, and reaches a maximum after 35–50 msec. 4. Comparison of the temporal characteristics of the burst of unit activity and of depression of the slow negative wave indicates that depression of the slow negative wave is not the result of desynchronization of cortical neurons caused by their excitation from the RF, but reflects direct depression of reciprocal cortical inhibition, as a result of which a secondary increase in the level of neuronal activity takes place.