In order to investigate the possible involvement of free polyamines and proline in the mechanism underlying the action of nitrate in correcting the negative effects associated with ammonium and urea nutrition in certain plant species, we studied plant contents of free polyamines and proline associated with nitrogen nutrition involving different nitrogen forms (nitrate, ammonium, urea) in two plant species, wheat and pepper. The results showed that ammonium nutrition and, to a lesser extent, urea nutrition were associated with significant increases in plant putrescine content that were well correlated with reductions in plant growth. These negative effects of ammonium and urea nutrition were corrected by the presence of nitrate in the nutrient solution; the presence of nitrate was also related to a significant decrease in the plant putrescine content. These results are compatible with a specific effect of nitrate reducing ammonium accumulation through the improvement of ammonium assimilation. As for the plant proline content, in pepper a slight increase in this parameter was associated with ammonium and urea nutrition, but it was also decreased by the presence of nitrate in the nutrient solution. These changes, however, were not so clearly related to the variations in plant growth as in the case of putrescine content. These results are compatible with the hypothesis that putrescine biosynthesis might be related to proline degradation by a specific pathway related to ammonium detoxification.