The objective of this study was to determine how the uptake and transport of nitrate by two species of arbuscular mycorrhizal (AM) fungi is affected by its concentration in the medium and by the age of the AM symbiosis. Tracer amounts of15N nitrate were applied at two plant growth periods to mycorrhizal or nonmycorrhizal lettuce plants, which had been grown in soil supplied with nitrate to provide a total of 84, 168, or 252 mg N/kg. At both injection times, Glomus mosseae (Nicol. and Gerd.) Gerd. and Trappe and Glomus fasciculatum (Thaxter sensu Gerd.) Gerd. and Trappe reached the highest values of nitrogen derived from the fertilizer (NdfF) at 84 mg N/kg. Glomus mosseae also reached the highest values of labeled fertilizer N utilization at 84 mg N/kg, whereas G. fasciculatum reached the highest values at 168 mg N/kg in the medium. The highest N level in the medium (252 mg N/kg) had a negative effect on % NdfF and % labeled fertilizer utilization for all mycorrhizal plants. Regarding the time of15N fertilizer application, G. fasciculatum-colonized plants had a minimum change in % NdfF and % labeled fertilizer utilization during the growth period (60 days application vs. 30 days application). In contrast, G. mosseae-colonized plants growing at 168 mg N/kg in the medium, decreased these two values in the latest application. The present results confirm that mycorrhizal symbiosis may be particularly important for nitrogen nutrition in plants growing in neutral-alkaline soils.Key words: arbuscular mycorrhizae, nitrate assimilation, nitrate uptake,15N-labeled fertilizer.