Ascorbic acid and polyamines were tested on a paraquat-susceptible rice mutant to evaluate the effect of exogenously applied chemicals on its susceptibility to this herbicide. The results showed that polyamines, especially spermine at 5 mM, could improve rice tolerance to paraquat. Although the ascorbate peroxidase activity was only slightly increased by spermine, a significant decrease of H2O2 suggested that this polyamine effectively improved paraquat resistance of rice plant through enhanced activity of antioxidative system.