The ages of three precambrian orogenic cycles have been defined hy means of U/Ph, Rb/Sr and K/Ar determinations carried out on igneous and metamorphic rocks of southeast Brazil. This work confirms the preliminary results obtained by the same authors on the granitoid rocks of Serra dos Órgãos (which belong to the Brazilian orogenic cycle, about 620 m.y.), and on the granulitic gneisses of the Paraíba do Sul Formation (Trans-Amazonian orogenic cycle, about 2070 m.y.). For the Mantiqueira gneisses, considered, by field relationships, to be the oldest rocks of the region, new results establish an age of about 2800 m.y. (λRb87 = 1,47 X 10- 11 years-1). The Juiz de Fora Formation has the same petrologic characteristics as the Paraíba do Sul Formation, and it is supposed to be discordant over the Mantiqueira gneisses. However, the geochronological results seem to demonstrate that it is, at least in part, as old as the latter formation. In addition, this work demonstrates a practically complete redistribution of Sr isotopes among the minerals of all rocks of the region, including the oldest ones, during the late precambrian Brazilian orogeny.The ages of three precambrian orogenic cycles have been defined hy means of U/Ph, Rb/Sr and K/Ar determinations carried out on igneous and metamorphic rocks of southeast Brazil. This work confirms the preliminary results obtained by the same authors on the granitoid rocks of Serra dos Órgãos (which belong to the Brazilian orogenic cycle, about 620 m.y.), and on the granulitic gneisses of the Paraíba do Sul Formation (Trans-Amazonian orogenic cycle, about 2070 m.y.). For the Mantiqueira gneisses, considered, by field relationships, to be the oldest rocks of the region, new results establish an age of about 2800 m.y. (λRb87 = 1,47 X 10- 11 years-1). The Juiz de Fora Formation has the same petrologic characteristics as the Paraíba do Sul Formation, and it is supposed to be discordant over the Mantiqueira gneisses. However, the geochronological results seem to demonstrate that it is, at least in part, as old as the latter formation. In addition, this work demonstrates a practically complete redistribution of Sr isotopes among the minerals of all rocks of the region, including the oldest ones, during the late precambrian Brazilian orogeny.Três ciclos orogênicos sucessivos, no Pré-Cambriano, foram definidos por meio de determinações U/Pb, Rb/Sr e K/Ar efetuadas em rochas ígneas e metamórficas da região sudeste do Brasil. Este trabalho confirma os resultados preliminares obtidos pelos mesmos autores para as rochas granitóides da Serra dos Orgãos (que pertencem ao ciclo orogênico Brasiliano, com cerca de 620 m.a.), e para os gnaisses granulíticos da Formação Paraíba do Sul (ciclo Transamazônico, cerca de 2070 m.a.). Para os gnaisses Mantiqueira, considerados a unidade mais antiga da região, novos resultados estabelecem uma idade de aproximadamente 2800 m.a. (λRb87 = 1,47 X 10- 11 anos). A Formação Juiz de Fora possui as mesmas características petrológicas que a Formação Paraíba do Sul, e estaria discordantemente sobreposta aos gnaisses Mantiqueira. Entretanto os dados geocronológicos parecem demonstrar que é, pelo menos em parte, tão antiga quanta aquelas rochas. Finalmente, os resultados demonstram uma redistribuição praticamente completa dos isotopos de Sr entre os minerais de todas as rochas da região, inclusive as mais antigas, durante o ciclo orogênico Brasiliano, no fim do Pré-Cambriano.