Effects of steroid hormones on the collagen biosynthesis in aorta and uterus were studied with ovariectomized Sprague-Dawley rats. Effects of administration of hormones, such as estrogen, testosterone and prednisolone, to the ovariectomized animals were studied, comparing with the control. Each group was injected with ³H-proline and sacrificed, followed by removals of aorta and uterus. Separations and quantitative analyses of proline and hydroxyproline were performed by means of thin layer chromatography; and radioactivities of the separated amino acids were assayed by liquid scintillation counter. Normally the incorporation of ³H-proline into hydroxyproline was greater in uterus than in aorta, and collagen turnover rate of uterus was observed rapid as well than that of aorta. In the two tissues from ovariectomized rats, the incorporation rate of ³H-proline into hydroxypoline was markedly decreased than that of the former. Changes in the turnover rate of collagen in these tissues were not observed. Decrease in ³H-proline incorporation into collagen in ovariectomized rats was markedly antagonized by estrogen, but not influenced by prednisolone in the tissues tested.