Cucumis sativus L. cv. Aminex (F1 hybrid) was grown alone or in symbiosis with Glomus intraradices Schenck and Smith in containers with two hyphal compartments (HC(A) and HC(B)) on either side of a root compartment (RC) separated by fine nylon mesh. Plants received a total of either 100, 200 or 400 mg N which were applied gradually to the RC during the experiment. N-15 was supplied to HC(A) 42 d after planting, at 50 mg (NH4+)-N-15-N kg-1 soil. Lateral movement of the applied N-15 towards the roots was minimized by using a nitrification inhibitor and a hyphal buffer compartment. Non-mycorrhizal controls contained only traces of N-15 after a 27 d labelling period irrespective of the amount of N supplied to the RC. In contrast, 49, 48 and 27% of the ap lied N-15 was recovered in mycorrhizal plants supplied with 100, 200 and 400 mg N, respectively. The plant dry weight was increased by mycorrhizal colonization at all three levels of N supply, but this effect was strongest in plants of low N status. The results indicated that this increase was due partly to the improved inflow of N via the external hyphae. Root colonization by G. intraradices was unaffected by the amount of N supplied to the RC, while hyphal length increased in HC(A) compared to HC(B). Although a considerable N-15 content was detected in mycorrhizal roots adjacent to HC(B), only insignificant amounts of N-15 were found in the external hyphae in HC(B). The external hyphae depleted the soil of inorganic N in both HC(A) and HC(B), while the concentration of soil mineral N was still high in non-mycorrhizal containers at harvest. An exception was plants supplied with 400 mg N, where some inorganic N was present at 5 cm distance from the RC in HC(A). The possibility of a regulation mechanism for hyphal transport of N is discussed.