Abstract. Using 13NOf-efflux analysis, the induction of nitrate uptake by externally supplied nitrate was monitored in roots of intact Picea glauca (Moench) Voss. seedlings over a 5-d period. In agreement with our earlier studies, efflux analysis revealed three compartments, which have been identified as surface adsorption, apparent free space, and cytoplasm. While induction of nitrate uptake was pronounced, NO3 fluxes in induced plants were decidedly lower and the induction response was slower than in other species. Influx rose from 0.1 gmol-g-t'h -1 (measured at 100 laM [NO3]o) in uninduced plants to a maximum of 0.5 gmol-g-1. h-1 after 3d of exposure to 100gM [NO~]o and declined to 0.3-0.4gmol-g-t-h -1 at the end of the 5-d period. Et~ux remained relatively constant around 0.02-,0.04 gmol.g-l-h-1, but its percentage with respect to influx declined from initially high values (around 30%) to steady-state values of 4-7%. Cytoplasmic [NO~-] ranged from the low micromolar in uninduced plants to a maximum of 2 mM in plants fully induced at 100 gM [NO~]o. In-vivo root nitrate reductase activity (NRA) was measured over the same time period, and was found to follow a similar pattern of induction as influx. The maximum response in NRA slightly preceded that of influx. It increased from 25 nmol.g-l-h -t without prior exposure to NO3 to peak values around 150 nmolg-1. h-1 after 2 d of exposure to 100 txM [NO3]o. Subsequently, NRA declined by about 50%. The dynamics of flux partitioning to reduction, to the vacuole, the xylem, and to efflux during the induction process are discussed.