Fine structural changes of rat duodenal absoptive cells after oral administration of ferrous chloride and sulfate were investigated and compared with those after calcium chloride administration.Male albino rats of WISTAR strain were fed with iron deficient diet and received nothing but distilled water for 24 hours before experiment. The animals were sacrificed at definite times after intragastric infusion of the above-mentioned chemicals. A portion of the duodenum 2cm distal from the pyloric ring was cut, and fixed in buffered osmium tetroxide solution, then dehydrated with alcohol or acetone, embedded in Epon and sectioned with a PORTER-BLUM microtome. Most thin sections were stained with uranium and lead acetate or lead nitrate, and examined by an HU-11A electron microscope.Mitochondria of the duodenal absorptive cells in normal control rats show no major differences in structure from those of other cells, and particulate matters, 200 to 300Å in diameter, are seen embedded in the matrix of the mitochondria as previously reported by many authors.The most remarkable change in many cells induced by 3mg of ferrous chloride is the appearence of numerous electron dense granules in the cristae and the limiting membrane of mitochondria. These granules are composed of numerous subunits which differ morphologically from ferritin. They increase in number the most after two hours, whereas after thifty minutes and three hours the number of granules is scanty. At thirty minutes many small particles were observed attached to both in and out side of the microvilli of some absorptive cells.Mitochondrial changes induced by 0.3mg. of ferrous chloride after 3 and 4 hours are for the most part similar to those by 3mg of ferrous chloride. At 6 hours some aggregations of ferritin-like particles in the cytoplasm are observed surrounded by endoplasmic reticulum or within the so-called cytcplasmic inclusion bodies.By the administration of 3mg of ferrous sulfate analogous dense granules are seen in the cristae mitochondriales.Similar dense granules are also demonstrated in the cristae mitochondriales of rats given calcium chloride, but such granules are never seen in those given sodium chloride.The dense granules observed in the cristae mitochondriales after administration of ferrous chloride, sulfate or calcium chloride are quite different in size, density, number and structure from other particulate matters in mitochondria reported by many authors. The dense granules in the present study are considered to have a close relationship to the iron absorption.From the above-mentioned results the author has attained the following conclusion. Iron is absorbed by microvilli of the duodenal absorptive cells and transported to mitochondria in the state of ferrous ion which can not be shown by electron microscope. Then, by oxidation and reduction within the mitochondrial membrane, this ferrous ion becomes to iron micelle which is demonstrable electron microscopically, and by the combination with apoferritin in the cytoplasm, the iron micelle might make ferritin. This is the author's conception about iron absorption mechanism.