The Tatoosh volcanic-plutonic complex, exposed in and near Mount Rainier National Park, Washington, is one of a series of Tertiary igneous bodies that are exposed along the axis of the Pliocene to Holocene High Cascades volcanic chain of the Pacific Northwest. It is possible that these Tertiary bodies are the eroded roots of an ancestral calc-alkalic arc. The Tatoosh complex was emplaced in an Eocene and Oligocene volcanic terrane that evidently was oceanic in character. Isotopic ages of zircons from plutonic, hypabyssal, and volcanic phases of the complex show that Tatoosh igneous activity, probably fed by repeated injections of magma from a deep source, spanned some 12 m.y. The earliest activity was emplacement of a dike and sill swarm about 26 m.y. ago. Subsequently, about 25 m.y. ago and again about 22 m.y. ago, magma broke through to the surface to form thick welded tuffs. Magmatism culminated with the rise of the core of the pluton (in part invading its own earlier volcanic and hypabyssal cover) in two stages, 17.5 and 14.1 m.y. ago.