Abstract: NADH-Glutamate synthase (GOGAT, E.C. 1.4.1.14) from Cucurbita pepo L. (marrow) cotyledons was inhibited by abscisic acid (ABA) at 10-6 M. Addition of either gibberellic acid (GA3) or dichlorophenoxyacetic acid (2,4-D) at 10-6 M to ABA counteracted its inhibitory effect. Kinetin at the same concentration increased inhibition of the enzyme by ABA. Cycloheximide, rifampicin, cordycepin and chloramphenicol at 0.3 M reduced mediated-increase in the enzyme activity by 2,4-D particularly in vivo. The enzyme was purified to homogeneity as determined by sodium dodecyl sulfate-polyacrylamide gels. The subunit Mr of GOGAT as determined by SDS-PAGE was 200 KDa. The specific activity was 131.2 U mg-1 protein. Group-specific modifiers like N-bromosuccinimide (NBS), Densyl chloride (DC), Tetranitromethane (TNM) and 2-ethoxy-1-ethoxy-1,2-dihydroquinoline (EEDQ) inactivated the purified enzyme. The results suggested the presence of tryptophanyl, lysyl, tyrosyl and carboxyl groups essential for the enzyme catalysis. Double logarithmic plots of the observed pseudo-first order rate constants against modifier concentration revealed modification of only one residue of each group.