K-Ar ages of biotites from the southern half (Atlanta Lobe) of the Idaho batholith are younger than the true emplacement ages of the rocks. For Mesozoic plutons with true ages probably greater than 100 m.y., the K-Ar ages range from 95 m.y. along the east and west margins of the batholith to 37 m.y. in the east-central zone: there is a conspicuous age gap between 46 and 53 m.y. However, a north trending group of epizonal Eocene plutons in the east-central zone has K-Ar ages of 44¿6 m.y., values which closely approximate the crystallization ages. These Eocene plutons, which were emplaced at shallow levels and underwent rapid cooling, are responsible for widespread (>10.000 km2) meteoric-hydrothermal alteration of the Mesozoic country rocks. Partial chloritzation and lowering of the K-O, deuterium, and 18O contents of the biotites accompanied the hydrothermal activity. Although the intensity of the hydrothermal effects is geographically correlated with the degree of lowering of the K-Ar ages, the production of anomalously young K-Ar ages is attributed in part to regional uplift and doming in addition to the hydrothermal metamorphism. In any given part of the Atlanta Lobe, the K-Ar ages of the higher elevation samples become systematically older with increasing elevation. This effect is observed in almost all of the biotites, both those that have undergone intense hydrothermal alteration and those that show no evidence of any kind of hydrothermal effects. The only exceptions are biotites that underwent complete Ar loss: such samples occur only at low elevations and within the contact metamorphic aureoles of the Eocene plutons. Age versus elevation trend lines are linear, with slopes that indicate uniform late Cretaceous and early Tertiary uplift (~0.10-0.14 mm/yr) and cooling of the terrane. The trend lines have almost identical slopes throughout the region, but the positions of the trend lines shift systematically upward to higher elevations from west to east, indicating that subsequence decreasing and faulting of the terrane modified the relative elevations of different subregions. The zone where this more recent uplift is most prominent almost coincides with the axis of the Eocene intrusions. Furthermore, the K-Ar age versus elevation trend lines are systematically truncated both at low elevations and close to contacts with Eocene plutons, apparently because of a temperature increase due to widespread hydrothermal heating of the Mesozoic rocks by the Eocene batholiths. In rocks located at present-day elevations below about 1000-1500 m in the central part of the Atlanta Lobe, biotites lost practically all their radiogenic Ar during the Eocene intrusive event.