Cell specific methylation patterns and levels are maintained stable in the soma. At the inverse, dramatic changes of DNA-methylation have been reported both in embryo proper and in germ cells. Most of the studies devoted to this subject used molecular techniques in order to detect changes occuring in DNA of house keeping, tissue specific or imprinted genes. With those techniques it is impossible to get a synthetic view of the methylation state at the chromosomal level, lack of information damaging to further understand phenomenons occuring during gametogenesis. In this aim, we applied an in situ approach using monoclonal antibodies raised against 5-methylcytosine (5-mC) in order to follow these changes during the initial phase of mouse spermatogenesis. In this purpose, cytogenetic preparations were successively stained by giemsa, DAPI and anti-5mC antibodies and then pictured. Results indicated the presence of two cell populations at that time : Sertoli cells and gonocytes. Probably in Sertoli cells, a pattern of methylation very similar to the one observed in somatic cells was evidence at all stages. In interphasic nuclei, it is characterized by 5-mC rich patches corresponding to heterochromatic regions intermingled with euchromatin showing few 5-mC sites. The corresponding metaphases showed rich 5-mC heterochromatin with a poor 5-mC euchromatin. The other population which has a timing of replication similar to the gonocyte's one, shows changes in its methylation pattern along the different developmental phases. At 15 dpc, metaphases and nuclei presented a complete demethylation. At 18dpc, this population was no longer visible. Another one was observed, composed by large nuclei showing a diffuse and homogeneous hypermethylated chromatin. No corresponding metaphases were observed. Two days after birth, hypermethylated nuclei were still seen there, mixed with a few hypermethylated metaphases. The methylation pattern of metaphasic chromosomes is very peculiar. In most of the metaphases the two sister chromatids are hypermethylated whith no methylation in the heterochromatic regions. In others, a methylation asymetry of the sister chromatids is observed while no methylation of the heterochromatin could be seen. The role and significance of these changes will be discussed further.