Abstract
1. In order to clarify the manner in which maternal nursing modifies the circadian rhythm of plasma corticosterone in blind pups, specified schedules of maternal nursing were imposed on blind pups and the hormone rhythm was followed individually. 2. Two groups of blinded pups were continuously nursed, alternately by two mother rats having different circadian phases of lactating period. In both groups, phase angles of the original mother were predominantly reflected in the phase of the pups' hormone rhythm. 3. When the maternal nursing was limited only during the light period (L nursing), it caused no detectable effect on the pups' hormone rhythm. 4. In contrast, when the maternal nursing was limited only during the dark period (D nursing), approximately one half of the pups at the fourth post-natal week showed a bimodal pattern of hormone rhythm. A second peak appearing 180 degrees out of phase to the first peak disappeared by the sixth or eight post-natal week. 5. It is concluded that the circadian oscillation underlying the plasma corticosterone rhythm starts to function prenatally and the original mother (probably, a prenatal LD cycle) primarily determines the phase angle of the circadian oscillation in blind pups. The maternal nursing does not entrain the pups' circadian oscillation, but does modify transiently the overt hormone rhythm, probably through a discord between maternal nursing and pups' feeding phases.