Changes in prolyl hydroxylase activity and immunoreactive protein were studied in various chick embryo tissues during the embryonic development. Both the enzyme activity and the amoung of immunoreactive protein increased till the 16th day of development and declined thereafter in all tissues studied. Comparison of the enzyme activity to the content of the total immuno-reactive protein indicated that there are distinct differences in the degree of enzyme activity between different chick embryo tissues, and in the same tissue between different stages of embryonic development. The highest relative enzyme activities were found in cartilage and skin, in which about 60% of the enzyme was active on the 16th day of development and only 20-30% was active on the 20th day of development; the lowest values were observed in spleen and large vessels, in which below 10% of the enzyme protein was in the active form on the 20th day of development Gel filtration studies demonstrated that in cartilage of 16-day-old chick embryos about 60% of the total immunoreactive enzyme in the tissue was present in the form of active prolylhydroxylase tetramer, whereas on the 20th day of development only 30% of the enzyme protein in cartilage was in the tetramer form. By contrast, in large vessels of the 16-day-old chick embryos, essentially all the enzyme was in the form of prolyl hydroxylase monomers.