OBJECTIVE. To examine the protective effect of bone marrow mesenchymal stem cell (BMSC) transplantation against lung injury induced by paraquat (PQ) poisoning. MATERIALS AND METHODS. Fifty-four female SD rats were randomly divided into four groups: a PQ group, a BMSC treatment group, a BMSC control group, and a normal control group. BMSC, isolated and cultured in vitro, were injected into the tail veins of the rats. The mortality rate, the ratio of lung wet/dry weight, plasma levels of IL-1β, tumor necrosis factor-α (TNF-α), malonaldehyde (MDA), superoxide dismutase (SOD), glutathione peroxidase (GSH-P(X)), and NF-κBp65 expression in the lung tissues were examined during 14 days after injection. Results. Survival in the PQ group ranged from 8 to 12 days, with an average of 9.6 days. All the rats in the BMSC treatment group survived for more than 14 days. In the BMSC treatment group, the ratio of lung wet /dry weight (P = 0.02 < 0.05), plasma IL-1β and TNF-α levels (P = 0.00 < 0.01), MDA levels (P = 0.03 < 0.05), and expression of NF-κBp65 in the lung tissues (P = 0.00 < 0.01) were significantly lower than those in the PQ group by day 7 post-injection. However, the SOD and GSH-P(X) levels began to increase (P = 0.00 < 0.01) compared with the PQ group. There was no significant difference between the BMSC control group and the normal control group (P > 0.01). DISCUSSION AND CONCLUSION. BMSC transplantation may have the potential to protect against PQ-induced lung injury, by reducing lung edema and lipid peroxidation, inhibiting the release of inflammatory mediators. However, the protective effects require time to develop, and early BMSC transplantation may be more evident during the middle and late stages of therapy.