In mice of both sexes ranging in age from five days to two years, the development and time course of diurnal rhythm and light-related variations in pineal glycogen were systematically studied by a semiquantitative histochemical method. When the animals were maintained under the usual diurnal lighting conditions (12L:12D), the diurnal rhythm and light-related changes in pineal glycogen appeared first at 22 days of age and persisted until two years. The glycogenic response was most prominent between 60 and 150 days, and declined slightly in mice older than one year.
The glycogenic response and cell size tended to differ in the distal, middle and proximal portions of the body of the pineal. These regional differences changes with the ages of the animals.
When mice were kept in continuous darkness for seven days starting from 15, 23 or 60 days of age, the pineal showed a diurnal rhythm in pineal glycogen.
When mice were kept in altered light regimens such as continuous darkness, continuous lighting or 6L:6D from birth up to 30 days, no diurnal variation in pineal glycogen developed.