The phase angle of circadian rhythm of plasma corticosterone in individual blind pups was followed in order to find out an effective zeitgeber(s) during the post-natal development. 2. The retinal light pathway before the opening of eyes was not essential for the entrainment of pups' rhythm to the environmental light-dark (LD) cycle. 3. In blinded pups whose eyes were enucleated on the day of birth, no evidence was obtained for effective extra-retinal pathways for photic inputs. 4. Free-running circadian hormone rhythm of blind pups was not entrained by 'social contact' with other sighted pups in the same cage. 5. Even when the duration of nursing was varied from 2 to 4 weeks, the phase angle of pups' rhythm at the fourth week was not affected. 6. In mother-pups exchange experiments in which pups born from LD mother were reared by DL mother (LD-DL pups) and conversely, pups from DL original mother were reared by LD foster mother (DL-LD pups), about one half of the pups at the fourth post-natal week of both groups exhibited phase angles belonging to their original mothers and the other half showed phase angles of their foster mothers. 7. It is suggested that blind pups may retain the phase angle of circadian rhythm of the original mother and the overt hormone rhythm may be modifiable by the foster mother during the early phase of post-natal life.
