The effects of the state of arousal on photically evoked blink reflex were studied. Eyelid potential (ELP), the averaged electromyographic activity of the orbicularis oculi muscle, was used to define the onset latency and intensity of the reflex. ELP was maximum with the shortest latency when the subjects were awake and doing mental arithmetic. ELP amplitude gradually decreased and the latency lengthened with the advance in non-rapid eye movement (non-REM) sleep stages. In the REM stage, ELP was augmented and resembled that when awake. ELP is useful as an objective sign of the photically evoked blink reflex, which closely reflects cortical activities and brain stem function.