Chronic arthritis is a catabolic state that induces a decrease in body weight gain and skeletal muscle atrophy. The chronic phase of the disease is associated with the inhibition of the insulin like grow factor I (IGF-I) and the activation of the E3 muscle specific ubiquitin ligating enzyme muscle atrophy F-box, also named atrogin 1. Myostatin is a negative regulator of muscle growth that is increased in several situations of muscle wasting such as AIDS and liver cirrhosis-induced cachexia. The aim of this work was to analyze the effect of adjuvant-induced arthritis on muscular myostatin and atrogin-1 gene expression, as well as on IGF-I, from the beginning of the clinical symptoms of the disease. For this purpouse, arthritis was induced in male Wistar rats by an intradermal injection of Freund’s adjuvant in the sole of the paw. The clinical symptoms of arthritis began 10 days after the injection, and the highest arthritis index was reached on day 22. Arthritic and control rats were humanly killed on days 10, 15 and 22 after the adjuvant injection. Arthritis induced a decrease in body weight gain and in serum level of IGF-I as well as in its gene expression in the liver (P<0.01). In the arthritic rats gastrocnemius relative weight was not modified on day 10, it decreased on day 15 (P<0.01), and it was markedly lower on day 22 (P<0.01). The muscular expression of myostatin decreased in the same way that gastrocnemius relative weight (P<0.05). The gene expression of atrogin-1 was significantly increased in the skeletal muscle of arthritic rats on all days studied, specially on day 15 (P<0.01). These data suggest that arthritis induced muscle loss can be mediated by changes in atrogin-1 and IGF-I, but noy by an increase in myostatin gene expression in the skeletal muscle.