Four durometers of a modified copolymer of styrene-ethylene/butylene-sty-rene were evaluated after in-vitro exposure to saline at 37°C (3, 6, 9, or 12 months) or in-vivo subcutaneous implantation (3 months). Specimens were evaluated for changes in mass, chemistry, and mechanical behavior. In-vitro exposure resulted in minimal moisture uptake by all durometers, no chemical alterations in any durometer, and slightly increased stiffness of the 90A durometer. All changes occurred within the first three months. In-vivo exposure resulted in a greater increase in mass, particularly of the 70A durometer. Chemical alteration in the form of an exaggerated carbonyl was observed in the 70A durometer only. These changes are indicative of possible lipid absorption. Mechanical changes included decreased stiffness of the 35A and 50A durometers, increased stiffness of the 90A durometer, and no change in the 70A durometer. These changes do not reflect the physical and chemical changes and are attributed to a balance between lipid and fluid absorption along with possible absorption of biological antiplasticizing agents or mineral oil leaching, or both.