The effect of sodium valproate on photosensitive epilepsy was studied. The range of flash rates of intermittent photic stimulation to which patients were sensitive (photosensitive range) was tested prior to treatment, and the reliability of this measure established by repeated tests on 70 patients. The photosensitive range was measured on 50 patients prior to and during treatment with sodium valproate. In 27 patients photosensitivity was abolished and in a further 12 patients photosensitivity was significantly reduced. A group of 167 patients followed without treatment did not show significant improvement over a 7 year period. Sixteen patients had drug treatment withdrawn, and in 7 months their photosensitive range had returned to its predrug level.