In the present studies with whole cells and extracts of the photosynthetic bacterium Rhodopseudomonas capsulata the rapid inhibition of nitrogenase dependent activities (i.e. N2-fixation acetylene reduction, or photoproduction of H2) by ammonia was investigated. The results suggest, that the regulation of the nitrogenase activity by NH~-in R. capsulata is mediated by glutamine synthetase (GS). (i) The glutamate analogue methionine sulfoximine (MSX) inhibited GS in situ and in vitro, and simultaneously prevented nitrogenase activity in vivo. (ii) When added to growing cultures ammonia caused rapid adenylylation of GS whereas MSX abolished the activity of both the adenylylated and unadenylylated form of the enzyme. (iii) Recommenccment of H 2 production due to an exhaustion of ammonia coincided with the deadenylylation of GS. (iv) In extracts, the nitrogenase was found to be inactive only when NH2 or MSX were added to intact cells. Subsequently the cells had to be treated with cetyltrimethylammonium bromide (CTAB). (v) In extracts the nitrogenase activity declined linearily with an increase of the ration of adenylylated vs. deadenylylated GS. A mechanism for inhibition of nitrogenase activity by ammonia and MSX is discussed.