A qualitative radioautographic analysis showed that mature decidual cells of the mouse are able to incorporate 3H-proline. After 1 hour silver grains are concentrated mainly over these cells although some of them can be found over collagen fibrils. After 2,6 and 24 hours there is a progressive increase of silver grains on the extracellular space most of them concentrated over thick collagen fibrils. These results strongly indicate that mature decidual cells of the mouse produce collagen and that they conserve a behavior of the fibroblast from which they originated.