The non-sunexposed skin of 12 healthy individuals was investigated under the electron microscope and the fine structure of normal elastic fibers was compared by different methods in 3 groups of age: 1--10 years, 30--50 years, and 60 years. 1. The elastic fibers of human skin may be semiselectively stained with silver tetraphenylporphyrine sulfate (Ag-TPPS), uranyl acetate, lead citrate and tannic uranyl acetate. They are composed of an amorphous matrix which is interwoven and surrounded by microfilaments. Elastic fibers are fully developed in early periods of age, first showing numerous microfilaments as structural glycoproteins, and then the deposition of the amorphous proelastin to be transformed into elastin. 2. The period of maturation is followed continuously by processes of physiological aging: a) decreasing number of microfilaments, b) appearing of electron-dense inclusions into the elastin matrix and c) fragmentation and disintegration of the fiber. 3. These changes indicate that aging of the fiber includes diminished synthesis of structural glycoproteins (microfilaments) and increasing content of osmiophilic groups, such as polar aminoacids, fatty acids and calcium salts. The fiber becomes thus more susceptible to the activity of elastase, and may totally disintegrate in older age.