Introduction. Rheumatoid arthritis (RA) is a chronic, systemic, autoimmune inflammatory disease of unknown origin, mainly affecting synovial joints and related structures, including the adjacent musculature, generating great disability and reduction in quality of life. Aim. This study was designed to investigate the effect of low-level laser therapy (LLLT) on gastrocnemius of Wistar rats subjected to an experimental model of RA. Material and methods. Forty male Wistar rats were used, separated into: acute and chronic, being subdivided into Control Group (CG): without intervention, Lesion Group (LG): submitted to lesion, Laser Control Group (LCG): without lesion and with treatment, and Laser Lesion Group (LLG): submitted to lesion and LLLT. The treatment with LLLT occurred in four points of the right knee, wavelength of 660 nm, energy density of 5 J/cm2, energy per point of 0.003 J. Morphometric analysis was performed using a 40x magnification photomicrograph and analyzed using the Image-Pro-Plus 6.0 program. Results. As result of the acute group there was a difference only for muscle mass, being higher in CG. For the chronic group there was significant difference for cross-sectional area, larger and smaller diameter, again with the control group obtaining higher values than the others, for the number of nuclei LG was lower than CG and LCG, but LLG was not different from any of them. Conclusion. It is concluded that treatment with LLLT was not very effective in reversing the harmful effects of RA on the gastrocnemius muscle.