We investigated if low level laser irradiation (LLLI) can reduce muscular fatigue during tetanic contractions in rats. 32 male Wistar rats were divided in 4 groups receiving LLLT doses of 0 (control group), 0.5, 1.0 and 2.5 J/cm2. Irradiation lasted 32, 80 and 160 seconds respectively with a fixed power density of 31.25 mW/ cm2. The total energy doses were 0.08, 0.2 and 0.4 Joules respectively. Electrical stimulation induced 6 tetanic muscle contractions in the tibial anterior muscle. Contractions were stopped when the muscle force fell to 50% of the initial value for each contraction (T50%). There was no significant difference between the 2.5 J/cm2 laser-irradiated group and the control group in mean T50%-values. Laser-irradiated groups 0.5 J/cm2 and 1.0 J/cm2 had significantly longer T50% values than the control group. The relative peak force for the 6th contraction in the laser irradiated groups were significantly higher at 92.2 % (SD +/- 12.6%) for 0.5 J/cm2, 83.2 %(SD +/- 20.5%) for 1.0 J/cm2 and 82.9 %(SD +/- 18.3%) for 2.5 J/cm2 respectively, than for the control group, 50% (SD +/- 15%). Laser groups receiving 0.5 J/cm2 and 1.0 J/cm2, showed significant increases in mean performed work compared both to the control group and their 1st contraction values. Groups receiving laser irradiation with doses of 1 and 2.5 J/cm2 also showed significantly lower levels of Creatine Kinase (CK) in plasma than the non-irradiated control group. We conclude that these doses of LLLT inhibit development of muscular fatigue in rats.