The effects of simazine [2-chloro-4, 6-bis(methylamino)-s-triazine] on in vivo nitrogenase, glutamine synthetase, glutamate synthase, and glutamate dehydrogenase which are important in nitrogen assimilation of Rhodopseudomonas sphaeroides were investigated. Simazine inhibited the synthesis of nitrogenase and glutamine synthetase. The activity of glutamine synthetase in crude extracts of Rhodopseudomonas sphaeroides is less inhibited by simazine than that of glutamate synthase and glutamate dehydrogenase. These results suggest the possibility that simazine inhibits photosynthetic activity and so inhibits the synthesis of nitrogenase and glutamine synthetase in Rhodopseudomonas sphaeroides.