Effects of organic and inorganic nitrogen sources on growth and mineral nutrient concentrations of Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) Karst.) seedlings were compared in a 100-day experiment in a greenhouse. Seedlings were grown in pots containing peat. Nutrient solutions differing in ammonium, nitrate, arginine and glycine composition were supplied to the seedlings at three nitrogen (N) concentrations: 1, 3 and 10 mM. We used dual (13C, 15N) and single (15N) isotopic labeling to determine the uptake of organic and inorganic N at the end of the experiment. Seedling dry weights and mineral nutrient concentrations of the needles showed that both conifer species were able to grow well and maintain nutrient balance on all investigated N forms except for the ammonium-dominated nutrient mixtures at the 10 mM N concentration. In Scots pine, no significant differences in dry weights were found between seedlings grown on the amino acids and seedlings grown on a commercial fertilizer containing 61.5% NO(3-)-N and 38.5% NH(4+)-N. Isotopic labeling of seedlings indicated that uptake rates of arginine-N, glycine-N and NH(4+)-N were similar, and 7-8 times greater than uptake rates for NO(3-)-N in both species. In Scots pine seedlings, 100% of arginine-N, and at least 67% of glycine-N was derived from the uptake of intact amino acids through seedling roots or mycorrhizae. Corresponding figures for Norway spruce were 83% for arginine and 96% for glycine. The gas chromatography-mass spectrometry analyses confirmed the presence of intact labeled molecules of both arginine and glycine in seedlings. We conclude that arginine and glycine are comparable to inorganic N as N sources for growth of conifer seedlings.