Anabolic steroids are widely used to increase skeletal muscle (SM) mass and improve physical performance. Some dietary supplements also include potent steroid precursors or active steroid analogs such as nandrolone. Our previous study reported the anabolic steroid effects on SM in a castrated guinea pig model with SM measured using a highly quantitative magnetic resonance imaging (MRI) protocol. The aim of the current study was to apply this animal model and in vivo MRI protocol to evaluate the growth effects of four widely used over-the-counter testosterone and nandrolone precursors: 4-androstene-3 17-dione (androstenedione), 4-androstene-3beta 17beta-diol (4-androsdiol), 19-nor-4-androstene-3beta-17beta-diol (bolandiol) and 19-nor-4-androstene-3 17-dione (19-norandrostenedione). The results showed that providing precursor to castrated male guinea pigs led to plasma steroid levels sufficient to maintain normal SM growth. The anabolic growth effects of these specific precursors on individual and total muscle volumes, sexual organs, and total adipose tissue over a 10-week treatment period, in comparison with those in the respective positive control testosterone and nandrolone groups, were documented quantitatively by MRI.