SUMMARY Indices of structural change were examined in blood vessel walls subjected to increased tangential load in a new, in vitro model system. Ring segments of rabbit ear artery were maintained in organ culture medium for times up to 9 days. Tangential load was chronically applied with small, intraluminal springs made of 0.010 in. diameter stainless steel wire. The applied load was considered to produce lerels of circumferential wall tension corresponding to those induced by a range of levels of blood pressure. The indices of stnicturai change examined were the uptake of radioactively-labelled proline and thymidine, which indicate protein synthesis and cell division respectively. Increased uptake of both proline and thymidine was noted in artery segments under elevated mechanical tension after a latency of 3 to 4 days. The degree of uptake was related to the degree of calculated wall tension elevation. The work indicated that cell division and protein synthesis can be induced in the blood vessel wall by increased wall tension alone, in vitro.