In vertebrates, the basic brain neural networks are defined during embryonic development. The brain growth spurt occurs postnatally, accompanied by the rapid increases in cell number and brain volume. It remains unknown, however, how postnatal (post-hatch) neurogenesis contributes to the organization of neural network required for social behaviors. To address this subject, I focused on medaka fish (Oryzias latipes), which is a model animal for molecular genetics and show prominent post-hatch brain growth and behavioral development. First, I analyzed proliferation zones in the young (sexually immature) medaka brain and identified two zones that had not been identified in the adult brain. Next I intended to generate transgenic medaka fish in which post-hatch neurogenesis can be genetically modified. And I generated the transgenic line (HuC:loxP-DsRed-loxP-GFP) that HuC promoter drives specifically in newborn neural progenitors in the adult brain. Further when stochastic recombination was induced by micro-injection of Cre mRNA into the Tg embryos at the 1 cell stage, it resulted that visualization of clonally-related cells in compartmented regions in the telencephalon in the adult medaka brain. Also, heat induction of transgenic embryo (HSP:Cre) led to Cre-recombination in the nervous system. Interestingly, by using this both lines (HSP:Cre and HuC:loxP-DsRed-loxP-GFP), heat induction can induce different Cre-recombination pattern depending on the developmental stages when heat induction was performed. As a result of analyzing stochastic recombinated samples, I identified some clonal units in the telencephalon of adult brain. Finally, by using an infrared laser-evoked gene operator (IR-LEGO) system, induction of heat shock in a micro area in the developing brains led to visualization of clonally-related HuC-expressing cells in the adult medaka fish.