Abstract 1. 1. The photoreceptor's electrical response to light (ERG) in the crayfish, shows a clear rhythm of 23.5 hr when it is obtained in complete darkness (except for the test light pulses) and controlled ambient temperature. 2. 2. If an external light (zeitgeber) of moderate intensity and duration is applied during a free running recording, an advance, a delay or no change of the rhythm will be observed, depending on the moment of application of the pulse. 3. 3. The delays and advances plotted against the circadian time of pulse application, generate the “phase response curve”. The unimodality of the curve thus obtained is interpreted as an adaptative feature of the electroretinographic circadian rhythm. 4. 4. The existence of a circuit involved in the control of sensitivity to light of the oscillators responsible for the ERG circadian rhythm, is postulated.