The influence of simazine (2-chloro-4,6-bis(ethylamine)-1,3,5-triazine) on N03- uptake and nitrate reductase (NR) activity in wheat (Triticum aestivum) and cucumber (Cucumis sativus) seedlings was investigated. It was found that at first (up to 12 h after herbicide treatment) simazine enhanced and then inhibited NR activity in the leaves and roots of the tested plants. After seven days of growth in nutrient medium containing 2.5 and 25 µM simazine the herbicide inhibited by 25 and 70 per cent, respectively, NR activity and reduced by 30 per cent in wheat and 50 per cent in cucumber seedlings nitrate uptake. The probable mechanism of NR inhibition by simazin is discussed and it is suggested that the influence of the herbicide on NR activity may not be exclusively connected with photosynthesis inhibition.