EEG records of photosensitive epileptic patients with repeated EEG recordings at different seasons of the year, were examined for the occurrence of photoparoxysmal response to photic stimulation (photoparoxysmal discharges (PPD]. Recordings in summer showed a lower incidence of PPD compared with recordings in other seasons. Recordings in winter showed a higher incidence of PPD compared with recordings in other seasons. Details of the 11 patients, each of whom had EEG recordings in summer and in winter confirmed the relative rarity of PPD in summer compared with winter. This reflects a lower cerebral neuronal excitability in summer compared with winter. Mechanisms for this, though not clear, may be related to an increased amount of environmental sunshine during summer and/or reduced temperature during winter.