Tryptophan (TRP), the precursor of the scavenger or immunomodulator molecules melatonin (MLT) and picolinic acid, can be found in the diet; and could be an alternative nutritional supplement used to regulate the immune response in the generation of free radicals. In an experimental model, the systemic administration of lipopolysaccharide (LPS), to promote the synthesis of pro-inflammatory cytokines, reactive oxygen species, and antioxidant enzymes, was performed on adult female, pregnant and lactating rats fed with a diet of TRP content (0.5mg/100g protein). Lung tissue was evaluated for levels of the products of lipoperoxidation (LPO's), malonaldehyde (MDA) and 4-hydroxy alkenals (4-HDA); nitrites (NO2), glutathione peroxidase (Gpx) enzyme activity, and the serum concentration of interferon-gamma (IFN-gamma), which were measured in the following groups: control (CTRL), LPS, MLT, TRP, LPS plus MLT (LPS+MLT), and LPS plus TRP (LPS+TRP). Results showed that the lung tissue levels of MDA and 4-HDA in the LPS+TRP group were significantly lower than in the TRP group. Statistically significant differences were not observed in nitric oxide levels among the groups LPS+MLT and LPS+TRP compared to the group under endotoxic shock (LPS). The Gpx enzyme activity was modified in the LPS+MLT vs the LPS group, but the difference was not statistically significant. The LPS+MLT group showed a smaller serum concentration (98%) of IFN-gamma than the LPS group. Statistically significant differences were not observed among the animals of the LPS+TRP and the LPS groups. Copyright 2009 Elsevier Masson SAS. All rights reserved.