Antagonism of the activin receptor signaling pathway represents a promising potential therapy for the muscular dystrophies and other muscle wasting disorders (i.e., cachexia or sarcopenia). Previous research has shown that antagonism of activin signaling promotes muscle growth, attenuates muscle wasting, and restores function in both wild type and diseased animals. Our laboratory has recently developed a novel small molecule (SGI-1252) that inhibits activin downstream (i.e., Smad2/3 phosphorylation) signaling. Purpose: In this study we determined how eight weeks of orally administered SGI-1252 affected TGF-ß signaling, whole body mass, individual limb muscle mass, and muscle fiber cross sectional area (CSA). Methods: Wild-type (WT) mice were treated with SGI-1252 or a vehicle control (VC) via oral gavage (400 mg/kg 3 times per week) for 8 weeks. Body mass was measured twice per week during the 8-week treatment period. At the end of the treatment period, gastrocnemius and tibialis anterior (TA) muscles were excised, weighed, and prepared for histological and biochemical analyses. Results: Following 8 weeks of treatment, there was no difference in weight gain between SGI-1252 (24.8 ± 1.8g) and VC treated mice (23.2 ± 1.5g) (p = 0.06). Gastrocnemius whole muscle mass was significantly greater in the SGI-1252 treated group relative to the VC treated mice (139.6 ± 12.8 mg vs 128.8 ± 14.9 mg) (p = 0.04), although when normalized with body mass there was no difference in gastrocnemius mass. For the TA muscle, there were no significant differences in whole muscle mass between SGI-1252 and VC groups, yet TA muscles in the SGI-1252 treated group had a reduced muscle fiber CSA compared to controls (621 ± 44 µm2 vs 749 ± 36 µm2) (p = 0.0005). There was a statistical trend of decreasing Smad2 phosphorylation in the SGI-1252 treated TA muscles (mean SGI-1252 = 0.668 vs VC = 0.848) (p = 0.06), and no significant differences in Smad2 phosphorylation in the gastrocnemius. Conclusions: Contrary to our hypothesis, 8 weeks of orally administered SGI-1252 was not effective in promoting increases in whole body mass, limb whole muscle mass, or myofiber cross sectional area. This may be due to the inability of SGI-1252, at the administered dose, to effectively decrease signaling downstream of the activin receptor. Clearly, studies using a wider range of doses and delivery methods will be needed to ascertain the efficacy of SGI-1252 as a potential therapeutic.