Endothelial cells (EC) are constantly subjected to pulsatile stretch in vivo, but most studies of EC are performed under stationary tissue culture conditions. The aim of this study was to examine the effect of repetitive mechanical stretching on EC collagen production. Bovine EC were seeded in 35-mm flexible-bottom culture wells, allowed to attach for 24 hr, and then subjected to up to 24% elongation cyclic deformation, 3 cycles/min, 10 sec of elongation alternating with 10 sec of relaxation, for 5 days. Twenty-four hours prior to harvesting, serum-free media containing 50 microCi [3H]proline (PRO), an amino acid hydroxylated (OH) in collagen, and 50 micrograms/ml ascorbate were added per well. On Days 1, 3, and 5, the media and cells were collected, precipitated with TCA, sedimented, lyophilized, and analyzed by HPLC for OH-PRO and PRO. The results of this study indicate that EC production of collagen is inhibited with repetitive deformation. Since previous studies have shown that EC proliferation is increased after 3 cycles/min stretching, this supports the theory that collagen gene expression varies inversely with the proliferative state.