In the legume Phaseolus vulgaris L., glutamine synthetase (GS; EC.6.3.1.2.) is encoded by four actively transcribed genes, gln-Ξ±, gln-Ξ², gln-Ξ³ and gln-Ξ΄. We have studied the expression of these genes in cotyledons during seed germination and have studied the effect of light and nitrate on this process. An RNase-protection method, used to detect the abundances of GS mRNAs, revealed that the four GS genes are differentially expressed in the germinating cotyledons. The gln-Ξ±. mRNA was present in dry seeds and was the most abundant GS mRNA during early stages of germination. The gln-Ξ² and gln-Ξ³ mRNAs were first detectable 2 d after sowing and their abundances differed in light- and dark-grown cotyledons at later stages of germination. The gln-Ξ΄ mRNA (which encodes the plastid-located GS) was detectable only in light-grown cotyledons, at a low abundance. A nitrate supply of 2 mM had only a minor effect on the expression of the GS genes. Western immunodetection and ion-exchange high-performance liquid chromatography demonstrated that the Ξ± polypeptide and isoenzyme were present in extracts of dry seeds and represented the major GS products at 2 d and 4 d. Both the Ξ² and Ξ³ polypeptides appeared at the 2-d stage. The role of differential GS gene expression in controlling cotyledonary GS activity is discussed.