We assessed the hormonal status of adult female volunteers before and during a 3-wk period of weight gain induced by mixed diet overfeeding. Forty-six percent of the 4.3-kg average weight gain experienced by these subjects consisted of lean body mass (LBM) and it is of interest that there were also increases in plasma Somatomedin-C/Insulin-like Growth Factor (SM-C/IGF-1) and testosterone concentrations as well as insulin. We suggest that it was the combined anabolic effect of these three hormones that facilitated the increase in LBM. Of the other assays done, increases were recorded for urinary 17-ketosteroids, 17-hydroxysteroids, epinephrine, and creatinine, whereas there were no changes in serum cortisol or triiodothyronine (T3), or urine norepinephrine; serum thyroxine (T4) fell slightly. Thus it appears that energy surfeit as well as energy deficit (reported by others) has an effect on blood hormone concentrations.