The ERG evoked by achromate stimuli was used to study the properties of the parietal eye of the Green Iguana. The response consisted of a positive component (positive on-response and a negative off-response), and a negative component (negative on-response and no response at off) which were independent and had different properties. The negative component was more sensitive and had a long refractory period whereas the positive component had a higher threshold but had a very short refractory period. The parietal eye of the iguana studied had properties which were more like those of the lateral eyes of nocturnal reptiles (gecko and caiman) than those of its own lateral eye.