This paper presents ten new K-Ar analyses of rocks and minerals of the Precambrian basement of the region located to the NW of the city of São Paulo. Together with 21 analyses presented by Cordani and Bittencourt (1967), these data are discussed on basis of more recent geologic knowledge of the area . The highest values of the K-Ar ages obtained, here supposed minimum ages for rocks or geologic events, can be summarized as follows: - Post-kinematic granites (Itu type) - 560 m.y. - Late to post-kinematic granites (Anhangüera and Varejão types) - 613 m.y. - Late-kinematic granites (Pirituba type) - 645 m.y. - Metamorphism of São Roque Group - 714 m.y. - Jundiai Block - 1036 m.y. - Cotia Block - 1360 m.y. The K-Ar ages related to the granitic rocks are consistent with the geologic interpretation. The São Roque Group had been involved by metamorphic events with 714 m.y. minimum age and this fact indicates that it was affected by the Brasilian Orogenic Cycle on its early phase. The Jundiai and Cotia Blocks, made up of more deeply metamorphosed rocks, are supposed to be older. Isolated age measurements agree with this hypothesis and suggest that the older basement was remobilized during the Brasiliano Cycle.This paper presents ten new K-Ar analyses of rocks and minerals of the Precambrian basement of the region located to the NW of the city of São Paulo. Together with 21 analyses presented by Cordani and Bittencourt (1967), these data are discussed on basis of more recent geologic knowledge of the area . The highest values of the K-Ar ages obtained, here supposed minimum ages for rocks or geologic events, can be summarized as follows: - Post-kinematic granites (Itu type) - 560 m.y. - Late to post-kinematic granites (Anhangüera and Varejão types) - 613 m.y. - Late-kinematic granites (Pirituba type) - 645 m.y. - Metamorphism of São Roque Group - 714 m.y. - Jundiai Block - 1036 m.y. - Cotia Block - 1360 m.y. The K-Ar ages related to the granitic rocks are consistent with the geologic interpretation. The São Roque Group had been involved by metamorphic events with 714 m.y. minimum age and this fact indicates that it was affected by the Brasilian Orogenic Cycle on its early phase. The Jundiai and Cotia Blocks, made up of more deeply metamorphosed rocks, are supposed to be older. Isolated age measurements agree with this hypothesis and suggest that the older basement was remobilized during the Brasiliano Cycle.