Learning that occurs during the sensitive, or critical, period is believed to lay the foundation for future learning.　 Filial imprinting predisposes precocial birds to form social attachment to other animals or objects during a sensitive period restricted to the first few days after hatching.　 I will focus on the imprinting as an example of early learning and propose its novel physiological role during the sensitive period of learning:　 the priming role that leads to the acquisition of later learning, a subject that has not been discussed in detail since the first report of imprinting-like behavior by Spalding in the 19th century.　 A major point of discussion is the determining factor that starts and ends the sensitive period of imprinting; very little was known about the existence of a determining factor.　 I show that a rapid inflow of thyroid hormone from plasma critically determines the start of the period.　 This hormone (T3) also reopens a once closed sensitive period and enables imprinting again.　 Moreover, it confers what I term “memory priming” (MP) to prime subsequent learning, i.e., reinforcement learning is greatly facilitated by priming with T3.　 Once chicks have achieved MP, it is maintained for long periods, perhaps even for an unlimited period of time, via a non-genomic action.　 This supports my idea that MP which originates from imprinting is followed by cascading layers of later learning.　 In other words, learning itself (imprinting) opens the door to start the sensitive period, and the sensitive period can be restarted even after it has once closed.　 The novel role of imprinting in my study that primes learning at a later developmental stage is quite different from the classical view, which is restricted to the very specific learning of imprinting. This novel idea of imprinting will be discussed in my talk.