Fig. 2. Validation of RU486 as a corticosterone antagonist in the Gambel's white-crowned sparrow. Changes in fat score and plasma corticosterone (ng ml^-1) were compared among three treatment groups: (1) birds with empty implants and vehicle injections (controls), (2) birds with implants of corticosterone and vehicle injections (B group) and (3) birds with implants of corticosterone and injections of 50 mg kg^-1 RU486 (B+RU486 group). Corticosterone concentrations were determined from blood samples collected in under 3 min of disturbance. Corticosterone implants were effective in elevating plasma corticosterone levels, and caused an increase in fat score. RU486 treatment inhibited corticosterone-induced fat deposition. Nine animals were included in each treatment group. Error bars represent S.E.M. Different letters above bars indicate significant differences among groups.