This report describes experiments designed to study changes in metabolic patterns associated with the senescence of human amnion cells in vitro. Senescence has been defined as the increase in the chronological age of a population of cultured cells which are predominantly nondividing. This definition is based on the concept formulated by Comfort and Shock.1 The general scheme of these experiments consists of the determination of radioactivity in various cell components obtained from cultures previously labeled with C14 substrates. Materials and Methods Amnion Culture.— Cell suspensions were prepared from amniotic membranes carefully dissected from placentae obtained at normal full-term deliveries.2 Approximately 5 million cells suspended in 10 ml. of nutrient medium, consisting of 10% human and 5% calf or 20% horse serum in Eagle's basal medium,3 were implanted in milk dilution bottles of 200 ml. capacity. The media were renewed on the 4th and 7th days. On the