Woong Kim, ME; Chris E. Kawcak, DVM, PhD; C. Wayne McIlwraith, BVSc, PhD; Elwyn C. Firth, BVSc, PhD; Brian H. McArdle, PhD; Neil D. Broom, PhD American Journal of Veterinary Research May 2009 70 5 589-598 Objective—To investigate the influence of early conditioning exercise on the development of gross cartilage defects and swelling behavior of cartilage extracellular matrix (ECM) in the midcarpal joint of horses. Animals—12 Thoroughbreds. Procedures—6 horses underwent early conditioning exercise from birth to 18 months of age (CONDEX group), and 6 horses were used as control animals (PASTEX group). The horses were euthanized at 18 months of age, and the midcarpal joints were harvested. Gross defects of the cartilage surface were classified and mapped. Opposing surfaces of the third and radial carpal bones were used to quantify swelling behavior of the cartilage ECM. Results—A wide range of gross defects was detected in the cartilage on the opposing surfaces of the bones of the midcarpal joint; however, there was no significant difference between the CONDEX and PASTEX groups. Similarly, no significant difference in swelling behavior of the cartilage ECM was evident between the CONDEX and PASTEX groups. Conclusions and Clinical Relevance—In the study reported here, we did not detect negative influences of early conditioning exercise on the prevalence of gross defects in cartilage of the midcarpal joint or the quality of the cartilage ECM as defined by swelling behavior. These results suggested that early conditioning exercise may be used without negative consequences for the midcarpal joint and the cartilage ECM of the third and radial carpal bones.