Mouse embryos were extracted with 0.5% Triton X-100 and subjected to cellulose acetate electrophoresis. In fertilized eggs, two forms of alkaline phosphatase (ALP), a slow-moving form and a fast-moving form, were observed. As cleavage proceeded, the fast-moving form disappeared, and the slow-moving form, the mobility of which was similar to that of the slow-moving form of the kidney, became gradually dominant up to the blastocyst stage (named 'embryonic' form). With blastulation, another fast-moving form showing a similar mobility to the lung ALP began to appear in blastocysts and showed a transient dominance in hatched blastocysts. After implantation, both the embryonic form and the fast-moving form gradually faded, and were eventually replaced by the new form, which may be named 'fetal form' in Day 7 embryos. These results clearly demonstrated that ALP activity does exist in embryos at all stages of preimplantation development. Moreover, the changes in multiple forms of ALP correlated with embryonic development may suggest that these multiple forms may have differential roles in the process of early development.