Data for the migmatitic paragneisses of the Northwest Adirondacks indicate fractionation of potassium and rubidium during metamorphism and migmatization; total rock KRb ratios are controlled to a significant degree by modal composition. Biotite-poor granitic veins and lenses show KRb ratios averaging 431 in contrast to the host gneisses which have an average KRb ratio of 271. Paragneisses from the zone of granulite facies metamorphism show significantly higher (mean 307) KRb ratios than those from the amphibolite facies (231), indicating the possibility of regional depletion in Rb relative to K under intensive metamorphic conditions.