High-pressure metamorphic rocks (eclogites and high-pressure granulites) occur along the entire length of the Norwegian Caledonides, and have formed from a variety of protoliths. In some cases, the relationship between protoliths, high-pressure rocks and their later retrogression products have been preserved in-situ. Fluid-inclusion data suggest a simple correlation between metamorphic grade and metamorphic fluid composition: (1) Eclogites and high-pressure granulites contain N2-bearing fluids (pure N2, or mixtures with CO2 or H2O, with XN2 > 5%). In some eclogite-facies rocks, CO2/1bN2 inclusions are associated with aqueous brine inclusions (ca. 30 wt% NaCl), the two compositions representing immiscible fluids at peak metamorphic conditions. (2) Granulite-facies protoliths and eclogites reworked in the granulite-facies contain pure CO2 or CO2-dominated fluids with less than 2.5% N2. (3) Rocks retrograded in the amphibolite facies contain H2O/1bNaCl fluids. Immiscibility between brine and anhydrous N2/1bCO2 fluid, and between anhydrous fluid and waterbearing aluminosilicate-melt have taken place in some eclogites. During high-grade metamorphism, nitrogen may be incorporated in minerals, as NH4 substituting for K, or it may occur as N2 in the free fluid phase. The partitioning of nitrogen between minerals and fluids depends upon the water activity and oxygen fugacity during metamorphism, low aH2O and/or high fO2 partitioning nitrogen to the fluid phase. A rock interacting with a carbonic fluid at granulite-facies PT conditions will be depleted in mineralogically bound nitrogen. In cases where the protoliths of high-pressure rocks have been through a previous, granulite-facies event, a local source for the nitrogen contained in high-pressure fluid is therefore unlikely.