Using radioisotope-labeled testosterone, the conversion of testosterone to estrogen in the rat placenta was studied with combined morphological and biochemical methods. The following results were obtained.1. That testosterone is actually converted into estrogen in the rat placenta was confirmed by thin-layer chromatography and thin-layer autoradiography. Electron microscope autoradiography showed that this conversion takes place in layer 2 of the placental labyrinth. Of the estrogens formed during 30min after the administration of testosterone, estrone was most abundant (54.8%), with estriol and estradiol-17β following (16.2% and 5.1% respectively).2. Electron microscope autoradiography showed that among the organelles of the placental labyrinth, the majority of the developed silver grains were in the smooth endoplasmic reticulum. Autoradiography of the fractions of the placental labyrinth also showed the largest number of silver grains in the microsome fraction.3. When the radioactivity of each homogenate fraction was measured, the microsome fraction showed the highest level of activity (with the exception of the soluble fraction).4. That the enzymes involved in the conversion of androgen to estrogen occur in the microsome fraction has been shown by previous biochemical studies. This study confirms it and further, for the first time by electron microscope autoradiography, provides morphological proof that the smooth endoplasmic reticulum is the site of aromatization of androgen.