Pigmented mice differ from frogs in that lighting regime has only a small effect on rod outer segment turnover. During 8 days in constant light or darkness, disc addition measured as total displacement of a radioactive band in rod outer segments of mice which has received injections of tritiated amino acids was never modified by more than 7% compared to cyclic light controls. Disc shedding continued in each lighting regime, and as judged from outer segment dimensions, disc shedding approximately balanced disc addition.