Plasma concentrations of adiponectin and leptin were measured in 23 mature Standardbred mares (age: 10+/-3 years) and 12 weanling fillies (10 Quarter Horse/Belgian crossbreds and two Quarter Horses; aged: 4+/-3 months) to test the hypothesis that adipocytokines are proportional to adiposity in horses. Rump fat thickness was measured using B-mode ultrasound and percent body fat (% fat) calculated using a published formula for the estimation of fatness in horses. Plasma adiponectin and leptin were determined using radioimmunoassay. In the absence of purified equine adiponectin or leptin, results were expressed as human equivalents (HE) of immunoreactive (ir) adipocytokines. Plasma ir-leptin HE concentration was positively correlated (r=0.543; P<0.001) with percent body fat and (r=0.556; P<0.001) to fat mass in all horses. The plasma ir-leptin HE concentration was lower (P=0.03) in weanlings (1.90+/-0.34 ng/mL HE) than in mature mares (3.47+/-0.50 ng/mL HE). The ratio of ir-adiponectin HE to ir-leptin HE was negatively correlated (r=-0.621; P<0.001) to percent fat and (r=-631; P<0.001) to fat mass in all horses. The ratio of ir-adiponectin HE to ir-leptin HE was higher (P<0.001) in weanlings (3.04+/-0.51) than mature mares (1.03+/-0.13). These data suggest that leptin is proportional while adiponectin is inversely proportional to adiposity in horses.