The retinofugal pathways of 2-, 3- and 4-week-old rats were studied by the Fink-Heimer method. Unequivocal terminal degeneration was found in the suprachiasmatic nuclei of the hypothalamus at 3 and 4 weeks of age but not at 2 weeks of age. Closer examination of the interval between weeks 2 and 3 revealed that terminal degeneration was first demonstrable at 17 days, the same time at which a significant diurnal variation in serum corticosterone was detected. (Endocrinology94: 1201, 1974)