The ERG complex of the compound eye of the yellow mealworm bettle Tenebrimolitor (L.) separates into two components with stimulus durations of longer than 100 ms. These are the ‘on’ and the ‘off’ effects. Above stimulus durations of 1 s the positive potential following the ‘on’ is faster and the positive ‘off’ is followed by a small negative one. The latent period of the ‘on’ is independent of the stimulus duration while for stimulus durations of longer than 300 ms the ‘off’ latency is coupled with the end of the stimulus. If measured after the extinction of the stimulus the ‘off’ latency is affected by the stimulus duration. The ‘on’ and ‘off’ amplitudes behave similarly for various stimulus durations. Both originate in the receptor cells. The Bunsen-Roscoe Law of photochemistry holds for stimulus durations of 10-40 ms. For a given intensity, the amplitudes diminished for durations longer than 300 ms.