Female albino rats were exposed during 3 weeks to various photoperiodic sequences and levels of illumination. As long as the light span of the photoperiod did not exceed 12L-12D, the damaging effects of the light, even when intense (3,000 lx), on the retinal were relatively limited. On the other hand, the 14L-10D photoperiodic regimen that is usually used in standard rat animaleries produced marked degenerative lesions on the photoreceptors under either 1,200 or 3,000 lx. Under lighting schedules of 21L-15D or 28L-20D, the photoreceptors were heavely injured by 1,200 or 3,000 lx. The degenerative process affected all the photoreceptive cells as well as their various components: rods, cones, nucleus and synaptic endings.