A glutamine synthetase has been localised in the chloroplasts of Vicia faba. The enzyme has requirements for Mg(2+) and ATP in the biosynthetic reaction and in addition will catalyse a γ-glutamyl transferase reaction in the presence of Mn(2+) and arsenate. The enzyme is inhibited by AMP, CTP, glycine and alanine. These results are discussed in relation to the possible chloroplastic synthesis of nucleotide bases. Estimations of glutamine amide-2-oxoglutarate amino transferase (oxido-reductase) have demonstrated only low levels of activity in the chloroplast extracts. This enzyme is generally active in organisms where GS has an assimilary role. It is coneluded that glutamine synthetase has a biosynthetic and not an assimilatory role in the chloroplast.