The mechanisms of biodegradation of poly (DL-lactide), poly (epsilon-caprolactone), and copolymers of epsilon-caprolactone with DL-dilactide, delta-valerolactone, and DL-epsilon-decalactone in rabbit were shown to be qualitatively similar. However, the rate of the first stage of the degradation process, non-enzymatic random hydrolytic chain scission, varied by an order of magnitude and was dependent on morphological as well as chemical effects. Weight loss was generally not observed until the molecular weight had decreased to 15,000 or less. Poly (DL-lactide) differed from the other polyesters studied, the rate of chain scission increasing after the commencement of weight loss. The rate of weight loss was greater and the period prior to weight loss was shorter when the comonomer content of copolymers of epsilon-caprolactone was sufficient to reduce the melting point of epsilon-caproate sequences to body temperature.