This study had two objectives: 1) Identification of the type of cell involved in intimal proliferation (endothelial cells, smooth muscle fibres or fibroblasts). 2) Determination of the origin. For this purpose a model was used which consisted of implantation in the rat of femoral arterial segments in the interstitial tissue. Sequential examination of tissue collected on days 1, 2, 3, 4, 5, 6, 7 revealed that proliferation was more marked if not solely present at the extremities of the segments. The proliferating cells were also in continuity with the extravascular granulation tissue, which would appear to indicate that the intimal proliferation in this model came from the cells of the granulation tissue and "muscular" differentiation occurred in contact with the endothelium.