The effect of NO2 fumigation on root N uptake and metabolism was investigated in 3-month-old spruce (Picea abies L, Karst) seedlings, In a first experiment, the contribution of NO2 to the plant N budget was measured during a 48 h fumigation with 100 mm(3) m(-3) NO2, Plants were pretreated with various nutrient solutions containing NO3- and NH4+, NO3- only or no nitrogen source for 1 week prior to the beginning of fumigation. Absence of NH4+ in the solution for 6 d led to an increased capacity for NO3- uptake, whereas the absence of both ions caused a decrease in the plant N concentration, with no change in NO3- uptake, In fumigated plants, NO2 uptake accounted for 20-40% of NO3- uptake, Root NO3- uptake in plants supplied with NH4+ plus NO3- solutions was decreased by NO2 fumigation, whereas it was not significantly altered in the other treatments, In a second experiment, spruce seedlings were grown on a solution containing both NO3- and NH4+ and were fumigated or not with 100 mm(3) m(-3) NO2 for 7 weeks, Fumigated plants accumulated less dry matter, especially in the roots. Fluxes of the two N species were estimated from their accumulations in shoots and roots, xylem exudate analysis and N-15 labelling, Root NH4+ uptake was approximately three times higher than NO3- uptake, Nitrogen dioxide uptake represented 10-15% of the total N budget of the plants, In control plants, N assimilation occurred mainly in the roots and organic nitrogen was the main form of N transported to the shoot, Phloem transport of organic nitrogen accounted for 17% of its xylem transport, In fumigated plants, neither NO3- nor NH4+ accumulated in the shoot, showing that all the absorbed NO2 was assimilated, Root NO3- reduction was reduced whereas organic nitrogen transport in the phloem increased by a factor of 3 in NO2-fumigated as compared with control plants. The significance of the results for the regulation of whole-plant N utilization is discussed.