Potassium-argon determinations on micas from Precambrian granitic rocks of the Quadrilátero Ferrífero, Minas Gerais, Brazil, suggest three ages of intrusion: 2400 m.y. determined from gneiss within the Bação complex; 1350 m.y. from rocks in the northern part of the Bação complex and also in a region 7 km north of nearest known Minas series metasedimentary rocks (Late Precambrian age); and 450–550 m.y. The last has been determined from (1) gneiss of the eastern part of the area, (2) granitized beds of the Minas series, and (3) granite between Minas beds and the 1350 m.y. granite in the western part of the area. Other ages, ranging between 595 and 1080 m.y., may represent the effects of a younger metamorphism on older granitic rocks that are found between the peripheral area of the 1350 m.y. granites and Minas metasedimentary rocks.