Rearrangement and expression of the T cell antigen receptor and the gamma genes during T cell ontogeny is a regulated process; the gamma genes are rearranged and expressed first, followed by the beta and then the alpha genes. Expression of both functional alpha and beta gene RNA first occurs at day 17 of gestation, along with the expression of T3 delta chain RNA. T cell antigen receptor gene rearrangements occur primarily or exclusively in the thymus, although some gamma gene rearrangements occur outside the thymus in fetal liver cells that may be committed T cell progenitors. There is no gross difference in the extent of beta and gamma gene rearrangements in the adult thymocyte subpopulations that were analyzed, despite the fact that some of these populations cannot respond to antigen and never emigrate from the thymus. Quantitative analysis of rearrangements in total adult thymocyte DNA shows that beta gene rearrangements generally occur on both chromosomal homologs, and that rearrangements occur preferentially to the J beta 2 gene segment cluster.