The responses of the five cytosolic-type glutamine synthetase (GS1) genes of maize to treatment of hydroponically grown seedlings with 10 mM KNO3 or 10 mM NH4Cl were analyzed. Non-coding 3' gene-specific hybridization probes and radioanalytic imaging were used to quantitate individual gene transcript levels in excised roots and shoots before treatment and at selected times after treatment. Genes GS1-1 and GS1-2 exhibited distinct organ-specific responses to treatment with either nitrogen source. The GS1-1 transcript level increased over three-fold in roots, but changed little if any in shoots. In contrast, the GS1-2 transcript level increased over two-fold in shoots, but decreased in roots after treatment. Increased transcript levels were evident at 4 h after treatment with either nitrogen source, with maximum accumulations present at 8 h after treatment with ammonium and at 10-12 h after treatment with nitrate. The GS1-3 gene transcript level showed little or no change after treatment with either nitrogen source. The GS1-4 gene transcript level remained constant in shoots of treated seedlings, whereas in roots, it exhibited relatively minor, but complex responses to these two nitrogen sources. The GS1-5 gene transcript is present in very small amounts in seedlings, making it difficult to analyze its response to metabolites in young plants. These results provide support for the possibility that different cytosolic GS genes of maize play distinct roles in nitrogen metabolism during plant growth and differentiation.