The purpose of this study was to determine ultrastructurally the effects of testosterone, which is an androgenic hormone, on the thymic epithelial reticular cells of young female rats with transmission electron microscope. One and two months old young female Wistar rats were injected 325,5 mg/kg of testosterone at every second day for a period of 21 days. Control and treated rats were killed by decapitation. Their thymuses were fixed with glutaraldehyde and postfixed with osmium tetroxide, embedded in epon 812. Thin sections were stained with uranyl acetate and lead citrate. Corticomedullar areas were examined by TEM. We have observed hypertropy and increasing number of cytoplasmic vacuoles, lipid droplets, damaged mitochondria in some epithelial reticular cells in one month old young female rats. These findings much more prominent because of physiological involution in two months old female rats.