Abstract: This experiment was conducted to study the spatio-temporal expression patterns of the Cdc25B gene in early chicken embryos. The Cdc25B gene fragment was gained by molecular cloning. RNA probes of the Cdc25B gene were produced by in vitro transcription and Cdc25B transcripts were detected by whole mount in situ hybridization in the various developmental stages of the early embryo. The results showed that mRNA of Cdc25B was confined to the central nervous system (CNS), somites and tip of limb bud. The hybridization signal was weak in the caudal neural plate. In early stages, distribution of Cdc25B mRNA was asymmetrical in Hensen's node. As embryonic development progressed, the hybridization signal of Cdc25B in the CNS and somites became progressively stronger. Cdc25B transcripts displayed a ventro-dorsal decreasing gradient in the closing neural tube However, the notochord was negative in the various development stages The results suggest that chicken Cdc25B expression patterns support different specific roles of this regulator in development of the CNS, somites and tip of limb bud in early and later stages. These different roles depend on concentration of Cdc25B. In addition, asymmetrical development of inner organs was regulated by Cdc25B and other genes. The mechanisms which Cdc25B regulates CNS development may act through involvement of Cdc25B in progress of the cell cycle, and even in regulation of cell proliferation and differentiation events.