Potassium-argon mineral ages and reconnaissance mapping of the central and southern Alaska Range and Aleutian Range indicate that there were three major plutonic episodes during the Mesozoic and Cenozoic. The first began in the Early Jurassic about 180 m.y. ago and continued for about 25 m.y. Plutons of this age form on arcuate belt more than 500 mi (800 km) long that extends from Becharof Lake to the Talkeetna Mountains. The Jurassic plutonic rocks are largely diorite and quartz diorite with minor granodiorite. Late Cretaceous and Tertiary plutons occur locally within this belt but are largely confined to the northern part of the Alaska-Aleutian Range batholith, where they form north-trending belts transverse to the earlier tectonic elements and locally extend out into w at was a more stable area bordering the earlier tectonic features. Composition of these plutons ranges from diorite through granite, but granodiorite and quartz monzonite predominate. Isolated granitic stocks intruded during the second episode also extend eastward into the central Alaska Range. The data suggest that this plutonic episode may be separated into Late Cretaceous (72-84 m.y.) and early Tertiary (50-65 m.y.) phases. The third episode is represented by middle Tertiary (34-40 m.y.) plutons that form a north-trending belt about 90 mi (145 km) long in the central part of the southern Alaska Range. These rocks are characteristically granites and quartz monzonites. Small plutons of middle Tertiary age are present locally in the Alaska Peninsula and the central Alaska Range. The chem cal data that have thus far been accumulated on the rocks of these three plutonic episodes suggest progressive enrichment in alkali constituents with decreasing age.