We have found conditions allowing direct observation of nuclei in live one‐cell mouse embryos by video enhanced fluorescence microscopy without perturbation of their development. We use the vital dye Hoechst 33342 and a high sensitivity video camera coupled to a digital image processor for registration and treatment. we can decrease by a factor 50 to 100 the dye concentration generally used and by a factor 1000 the irradiation level. Under these conditions, we have studied the structural changes of paternal chromatin just after fertilization and the progression of pronuclei during the first cell cycle with a special emphasis on the chromatin recondensation before syngamy.