The progression of pre-implantation mouse embryos through the first, second and third embryonic cell cycle was investigated cytofluorometrically. In contrast to most of the previous studies, the ova were spontaneously ovulated and the mating period of the ova donors was short (06.00-09.00 hours). The embryonic cells proceeded through the first, second and third cell cycle as a cohort. Thus it was possible to estimate the duration of the cell cycle phases directly from the DNA histograms. The length of the cell cycle phases differed between the embryos of the two different mouse strains. The most pronounced differences were found for the G2 + M phases (first cell cycle: 8 hr for Strain I and 5 hr for Strain II; second cell cycle: 11.5 hr and 14 hr respectively). However, in accordance with previous investigations, common features of the early pre-implantation cell kinetics were also observed: increasing length of the S phases from the first to the second cell cycle and very short G1 phases in the second and third cell cycle. The cell proliferation of the embryos of both strains after the third cell cycle was characterized by exponential growth. The proliferation rate was higher in Strain I embryos than in Strain II embryos (steeper increase of the growth curve). At the end of the pre-implantation development (hatched blastocysts), the growth curves of both strains decreased. The differences concerning the durations of the cell cycle phases and the proliferation rates are considered to be strain specific. It is suggested that the differences in the pre-implantation cell kinetics which have been described generally reflect the strain specificities more than different investigational methods and/or different grades of synchrony of early pre-implantation embryos.