Sodium-rich hydrothermal alteration is widely developed in Permian to Jurassic arc igneous rocks of the western United States, but it is rare in younger rocks. Na-rich alteration reflects a paleogeographic control on development. Two varieties, NaCa and Na, can be defined by mineralogic and compositional changes. Both are common in Permian to Jurassic arc plutonic and volcanic rocks and are accompanied by Fe and Cu ± Zn mineralization. Geologic relations and mineralogic, stable isotopic, and fluid-inclusion data indicate that the fluids responsible for sodium-rich alteration were isotopically heavy, moderately to highly saline fluids of marine, formation, and/or meteoric origin, with or without a magmatic component. The transition away from widespread Na ± Ca-rich alteration in the late Mesozoic corresponds to and is consistent with the change from an early Mesozoic marine and/or arid rifted arc setting to a fully emergent, continental arc environment in the Cretaceous.