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SUMM.'^RVFoliar uptake and release of inorganic nitrogen compounds were studied by immersing current-year shoots of Scots pine {Pinus syhestris L.) and Norway spruce [Picea ahies (L.) Karst] in either NH^"-or NOg'-rain solutions at different N concentrations. The effects of N form, N concentration and tree species on ion influx and efflux were investigated.Spruce shoots absorbed NH^"^ from the external solution. Uptake apparently occurred by diffusion rather than by H"' or base cation exchange as commonly accepted, and increased iinearly with NH^"^ concentration in the external solution. In contrast, pine shoots released NH^* to the external solution. The different reactions of spruce and pine may refiect species differences in physical and chemical properties or differences in tissue N concentration. If the latter is the case, a tree's N status may determine whether the canopy acts as a source or sink for NH,', influencing deposition rates to the needle surface. The results show that where NHj* concentration on the needle surface exceeds 4 mg t"', foliar uptake may make a significant contribution to N status. In the absence of N'H^^-base cation exchange, atmospheric inputs of NH^"^ to the canopy appear unlikely to be directly responsible for the nutrient deficiencies typical of Dutch forest decline.Neither spruce or pine shoots were able to utilize NO^" in the external solution and generally released NO3". Adverse effects resulting from foliar accumulation of wet-deposited NO3" appear unlikely. However, higher NO3" concentrations and longer residence times than simulated in this experiment may result in foliar uptake of NOj" in the field.