With recent advance in analytical techniques many reports have appeared on the assays of steroid hormones, but as to the blood progestin at present the majority of these reports are concerned only with progesterone from the same test specimen, and there is as yet no report on simultaneous assays of other progestins. Therefore, the author has designed an analytical method for analyzing simultaneously 5 fractions of blood progestin. This method consist of 1) deproteinization, 2) the extraction of steroids, 3) delipidization, 4) alumina column chromatography, 5) the impregnant thin layer chromatography with ethyleneglycol, followed by the color assay, and the results of study on the turnover rate at each step have given satisfactory findings. As for the identificative analysis; 1) analysis by color reaction, 2) gas-liquid chromatography as well as 3) by thin layer chromatography, all yielded satisfactory results. Using this method, the author measured blood progestins at the first, the second and the third trimester of pregnancy. The results have demonstrated that there are many cases where the measurement of 17α-OH-progesterone is not possible because of its minimal quantity, but the other four fractions all showed an increasing tendency along with the advance in pregnancy stage. However, there could be observed no clear-cut correlations between the concentration of these progestin fractions and the uterine muscle susceptibility to oxytocin, neither was there any correlation between the fraction ratios centering around Δ5-3β-ol-steroid dehydrogenase and isomerase and the fraction ratios centering around 20-reductase. It has been suggested that the study of the states in the uterine muscular layer needs to be carried out simultaneously with the study of blood progestin in future.