The effects of various frequencies of intermittent photic stimulation (1/7, 1/3.5, 1/1, 3/1, 6/1, 9/1, 15/1, 18/1, 24/1, and 60/1 sec) on photically evoked afterdischarges (PhADs), photic driving (PD), and behavioral activity (gridcrossings) were examined in rats. Results demonstrate that frequencies from 3/1 to 60/1 sec induced reliable increases in behavioral activity. Well elaborated PhADs were elicited only at frequencies of 1/7, 1/3.5, and 1/1 sec, but only when behavioral activity was at a minimum. While PhADs could be elicited during the frequencies of 3/1-, 6/1-, and 9/1-sec stimulation, the predominant pattern was one of PD. With frequencies of 15/1, 18/1 and 24/1, the cortical EEG pattern was primarily one of PD. These results suggest that PhAD suppression by fast intermittent stimulation, per se, occurs only at frequencies of 9/1 sec and higher, suppression at lower frequencies being a result of behavioral activation by intermittent stimulation.