1. Indices of collagen synthesis and degradation were developed in a chick corium system in vitro. 2. These indices were determined by quantitatively measuring non-diffusible and diffusible hydroxy[14C]proline in the tissues after a standard period of incubation. 3. Under these incubation conditions collagen metabolism of the corium appears to be stable for at least 180min. 4. The indices of collagen synthesis and degradation seem to reflect the conditions of collagen metabolism in vivo. 5. A rapid collagen turnover occurs in the corium of the 13–14-day embryo owing to an accelerated collagen degradation at that period. 6. Epidermal elements may influence the synthesis as well as the degradation of collagen.