AbstractIn this study, an ex vivo organ culture system that allows the application of controlled loads to the anterior cruciate ligament (ACL) was designed and used to characterize the influence of a step input in mechanical load on gene expression. A procedure for isolating bone–ACL–bone (B–ACL–B) complexes from rat knees was developed. After harvest and 24 hour culture, B–ACL–B complexes exhibited percentages of viability similar to that in intact ACLs (∼90%). Application of a physiologically relevant load of 5 N (superimposed on a 1 N tare load) resulted in changes in levels of mRNA encoding type I collagen. While levels of type I collagen mRNA significantly increased 32 ± 13% (mean ± standard errors of the mean (SEM)) over controls within the first hour of loading, levels decreased significantly to 44 ± 9% of control after 2 h. Displacements induced by the 5 N load were measured by video dimensional analysis. Calculated axial strains of 0.141 ± 0.034 were achieved rapidly during the first hour and remained essentially unchanged thereafter. These results demonstrate the feasibility of maintaining ligaments in organ culture and illustrate the time course expression of type I collagen following the application of a mechanical load. © 2002 Orthopaedic Research Society. Published by Elsevier Science Ltd. All rights reserved.