Abstract
The testes of hamsters exposed to short days (10 hr of light per day) regress within 13 weeks. Administration of 7.5% deuterium oxide to hamsters lengthens the period of free running circadian activity rhythms by 2.2% and prevents testicular regression during short-day exposure. This consistent with predictions derived from an external coincidence model for photoperiodic time measurement: Deuterium oxide changes phase relationships between the light-dark cycle and the circadian system, the hamster's daily photosensitive phase is stimulated with light during short days, and the testes remain large. Conservation of the period of circadian rhythms within narrow limits has adaptive significance for hamster photoperiodism and for the occurrence and phasing of the annual reproductive cycle.