Abstract:Pigmented rats were kept under different light/darkness patterns for 5–12 days. The response of the iris sphincter in enucleated eyes was tested with pilocarpine as a standard stimulus. The eyes were either decorneated or intact. The maximum response (no subsensitivity) was obtained after continuous darkness and the minimum (maximum subsensitivity) after continuous light. Six hours of light and one block of 18 hours of darkness per day caused distinct subsensitivity, but less than if the light was interrupted by six 3‐hour darkness periods. Further subdivision of the light into twelve 30‐min. periods made no difference. One 18‐hour block of light followed by 6 hours of darkness caused less subsensitivity than continuous light. Subdividing the darkness into six 1‐hour periods made no significant d