The impacts of increased N deposition (wet; NH4+ and dry; NH3) on ombrotrophic mire vegetation were compared in a two year dose response study. Vegetation from an ombrotrophic mire in SE Scotland was exposed in open-top chambers to a range of wet N deposition at 0 (de-ionised water), 8, 16, 32, 64 & 128kg N ha-1y-1 applied as NH4CI and dry N deposition as gaseous NH3 at concentrations of 2, 6, 20, 50 & 90 ug m-3. N concentrations in the foliage of all species showed a significant linear increase in response to dose when N was applied as wet deposited NH4+ in both 1999 and 2000. Comparison of the linear regression for foliar N versus NH4+ and NH3 in the dose-response relaitonships showed significant differences (p,0.05) between the form of N applied and %N concentration for two species in 1999 (C. vulgaris and P. commune) but not in 2000. There was a significant incrase in C. vulgaris shoot growth in the NH3 treatments compared to NH4+ treatments when expressed per unit deposited N. Effects of nitrogen deposited as gaseous NH3 are generally greater than those of wet deposited NH4+ on moorland species, when expressed per unit deposited N.