The occurrence of glutamine synthetase, glutamate synthase and glutamate dehydrogenase in a range of halophytic higher plants has been investigated. Ferredoxin-dependent glutamate synthase was found in shoot tissue of all the species examined. Root tissue glutamate synthase was active with both reduced pyridine nucleotides and reduced ferredoxin. The levels of enzymes present in the majority of species are consistent with the view that ammonia assimilation occurs via the combined action of glutamine synthetase and glutamate synthase. An increase in external salinity lowers the levels of glutamine synthetase and glutamate dehydrogenase of root tissue but increases the level of shoot glutamine synthetase.