It is generally accepted that every placental protein shows its characteristics vicissitude pattern during pregnancy and that alkaline phosphatases (ALP) are subdivided into three kinds of isoenzymes, i.e.: tissue-unspecific, intestinal and termplacental ALPs. In this study, the changes in ALP isoenzyme profile throughout the course of pregnancy was studied by biochemistry, enzyme- and immuno-histochemistry, and the following results were obtained. In early pregnancy, ALP activity was composed mostly of tissue-unspecific ALP isoenzyme, and its activity revealed a peak value around 10 weeks of pregnancy as in the case of the hCG serum concentration. At the end of the second trimester of pregnancy, instead of tissue-unspecific isoenzyme, ALP activity was mainly composed of term placental ALP isoenzyme, but a very small amount of the isoenzyme was proved to exist in the villi at the 6th week of gestation. The above results were confirmed by morphological studies such as enzyme-cytochemistry and immuno-cytochemistry. It was therefore concluded that at least two genes of ALP isoenzymes existed in the villi and that the regulation of gene activity changed during the course of pregnancy. Furthermore, since tissue-unspecific ALP isoenzyme showed a pattern of change similar to that of serum hCG, the existence of a developmental phase-specific gene set was postulated.