Aquatic treadmills improve range of motion and increase muscular strength in mature horses; however, effects of buoyancy on inflammation and cartilage metabolism in young horses are not well investigated. Therefore, thirty Quarter Horse yearlings of similar breeding were stratified by age, BW, and sex and randomly assigned to one of three treatment groups during a 140-d trial to evaluate the influence of aquatic vs. dry exercise on joint inflammation and cartilage metabolism in young horses transitioning to an advanced workload. Treatment groups included non-exercise control (CON; n = 10), dry treadmill exercise (DRY; n = 10), or aquatic treadmill exercise (H2O; n = 10; water at 60% wither height). Animals were housed in individual stalls and allowed turnout for a minimum of 10 h/d. During Phase I, DRY and H2O walked on treadmills 30 min/d, 5 d/wk from d 0 to d 112. Phase II represented transition to an advanced workload 5d/wk for 28 d (Table 1). Every 28 d following exercise, synovial fluid samples were collected and analyzed for prostaglandin E2 (PGE2), collagenase cleavage neopeptide (C2C), collagenase of type I and type II collagen (C1,2C), and carboxypeptide of type II collagen (CPII) using commercial ELISA kits. All data were analyzed using PROC MIXED of SAS, including random effect of horse within treatment, and repeated effect of day. Baseline treatment differences were accounted for using a covariate structure. There were no treatment day interactions of synovial inflammation or markers of cartilage metabolism; however, there was an effect of day for each selected marker (P > 0.03). Changes in biomarkers of cartilage turnover in horses exercised at the walk, whether dry or aquatic, could not be distinguished from horses with access to turnout alone. This indicates that there are no negative effects of buoyancy on cartilage metabolism in yearlings transitioned from aquatic exercise to 28-d advanced workload.