We previously determined that, under salt stress, a growth medium containing a high level of ammonium enhances oxidative damage in rice (Oryza sativa L.) seedlings. This suggested that, to be salt-tolerant, plants must assimilate excess nitrogen efficiently. Here, we investigated the effect of different ammonium concentrations on the chilling-stress sensitivity of rice seedlings. We also examined the relationship between leaf polyamine contents and stress tolerance under low temperatures (13/$8^{circ}C$ day/night). Chilling treatment resulted in a striking decrease in chlorophyll fluorescence (${Phi}II$) in the leaf blades. This was especially true for seedlings grown in a high-ammonium medium. Thus, the chilling-stress sensitivity of rice was heightened under excess N, similar to that noted in our earlier salt-stress experiments. Exposure to low temperature caused contents of putrescine and spermidine to increase in leaf blades under all ammonium concentrations, whereas the level of spermine declined under such treatment. The rate of increase in putrescine and spermidine contents was slower when the medium contained a high amount of ammonium, and a positive correlation was observed between leaf ${Phi}II$ values and leaf spermidine contents. These results imply that an adaptive mechanism operates in rice seedlings and that spermidine is of physiological importance in conveying chilling-stress tolerance.