In mustard (Sinapis alba L.) cotyledons, NADH-dependent glutamate synthase (NADH-GOGAT, EC 1.4.1.14) is only detectable during early seedling development with a peak of enzyme activity occurring between 2 and 2.5 d after sowing. With the beginning of plastidogenesis at approximately 2 d after sowing, ferredoxindependent glutamate synthase (Fd-GOGAT, EC 1.4.7.1) appears while NADH-GOGAT drops to a very low level. The enzymes were separated by anion exchange chromatography. Both enzymes are stimulated by light operating through phytochrome. However, the extent of induction is much higher in the case of Fd-GOGAT than in the case of NADH-GOGAT. Moreover, NADH-GOGAT is inducible predominantly by red light pulses, while the light induction of Fd-GOGAT operates predominantly via the high irradiance response of phytochrome. The NADH-GOGAT level is strongly increased if mustard seedlings are grown in the presence of nitrate (15 mM KNO3, 15 mM NH4NO3) while the Fd-GOGAT level is only slightly affected by these treatments. No effect on NADH-GOGAT level was observed by growing the seedlings in the presence of ammonium (15 mM NH4C1) instead of water, whereas the level of Fd-GOGAT was considerably reduced when seedlings were grown in the presence of NH4C1. Inducibility of NADH-GOGAT by treatment with red light pulses or by transferring water-grown * To whom correspondence should be addressed Abbreviations and symbols: c=continuous; D=darkness; Fd-GOGAT = ferredoxin-dependent glutamate synthase (EC 1.4.7.1); FR=far-red light (3.5 W'm-2); NADH-GO-GAT=NADH-dependent glutamate synthase (EC 1.4.1.14); Pfr = far-red absorbing form of phytoehrome; Ptot = total phytochrome; R=red light (6.8 W-m-Z); RG9-1ight =long wavelength FR (10 W-m -z, ~0RG9<0.01); q~0~)=Pfr/Ptot=wavelength-dependent photoequilibrium of the phytochrome system seedlings to NO3-containing medium follows a temporal pattern of competence. The very low Fd-GOGAT level in mustard seedlings grown under red light in the presence of the herbicide Norflurazon, which leads to photooxidative destruction of the plastids, indicates that the enzyme is located in the plastids. The NADH-GOGAT level is, in contrast, completely independent of plastid integrity which indicates that its location is cytosolic. It is concluded that NADH-GOGAT in the early seedling development is mainly concerned with metabolizing stored glutamine whereas Fd-GOGAT is involved in ammonium assimilation.