New-born rats were optically enucleated on day 1 and the rhythms of pineal serotonin N-acetyltransferase activity and serum corticosterone levels were followed under various nursing schedules from 4 to 8 weeks of ages. When blinded pups born under DL (LD) cycle were reared by foster mothers under LD (DL) cycle, both of N-acetyltransferase and corticosterone rhythms were in phase with those of the pups born of and reared by their original mothers under LD (DL) cycle. The phases of the rhythms were regularly delayed at a similar rate in 4 groups as they grew, indicating that nursing mother rats can entrain the circadian rhythms of blinded pups. When intact and blinded pups were reared under LD cycle, the phases of N-acetyltransferase activity rhythm in both groups were identical at the first postnatal week, but the phase of the rhythm in blinded pups was gradually delayed after second week compared to intact pups. The observation indicates that the endogenous oscillation in blinded pups starts to free-run between 1 and 2 weeks after birth.