Chromatin remodelling in the fertilised mouse egg is intimately linked to protein synthesis and degradation, to protamine by histone replacement and to specific histone modifications. The involvement of histone deacetylases (HDACs) in the beginning of development is poorly understood. HDACs are essential for cell proliferation and in the control of gene expression in a wide variety of mammalian systems. Here we focus on mHDAC6, a recently identified class II histone deacetylase, and we analyse its expression and localisation in oocytes and pronuclear zygotes. By indirect immunofluorescence we show that mHDAC6 is detected in the cytoplasm of germinal vesicle (GV) stage oocytes and 1-cell embryos. Ectopic expression of this enzyme after injection into germinal vesicles and pronuclei alters the nuclear structure and causes premature compaction of the chromatin. Our data suggest that the effect of condensation is linked to the ubiquitin-binding activity of mHDAC6, rather than to its function as a deacetylase.