Immature thymocytes that lack both Lyt-2 (CD8) and L3T4 (CD4) expression can respond rapidly to stimulation with phorbol ester and calcium ionophore by expressing some gene products characteristic of mature, activated T cells. Here we studied the effect of such short-term stimulation on the number of copies per cell of RNA for components of the T cell receptor complex. Although, upon stimulation, mRNAs for T cell receptor beta chain accumulated to higher levels, the cells did not rapidly increase their expression of alpha-chain transcripts from rearranged or germ-line genes. Transcripts from the C gamma 1 (C gamma 13.4) and C gamma 2 (C gamma 10.5) genes were differentially regulated. The rarer C gamma 1 transcripts were strongly induced, while the initially abundant C gamma 2 transcripts showed a modest decrease in transcripts per cell within 24 h. Thus, the ratio of these two transcripts could be shifted dramatically prior to any significant change in the cellular composition of the population. These results suggest regulatory processes that may contribute to the observed expression of gamma products in vitro or in normal development.