Regulated expression of the recombinase RAG-1 and RAG-2 proteins is necessary for generating the vast repertoire of antigen receptors essential for adaptive immunity. Here, a retroviral cDNA library screen showed that the stress-regulated protein GADD45a activated transcription of the genes encoding RAG-1 and RAG-2 in transformed pro–B cells by a pathway requiring the transcription factor Foxo1. Foxo1 directly activated transcription of the Rag1-Rag2 locus throughout early B cell development, and a decrease in Foxo1 protein diminished the induction of Rag1 and Rag2 transcription in a model of receptor editing. We also found that transcription of Rag1 and Rag2 was repressed at the pro–B cell and immature B cell stages by the kinase Akt through its 'antagonism' of Foxo1 function. Thus, Foxo1 is a key regulator of Rag1 and Rag2 transcription in primary B cells.