TCDD is known to induce thymic atrophy in several mammalian species through activation of programmed cell death, or apoptosis. To investigate the time course of events which precede TCDD-induced thymic apoptosis in vitro, experiments were performed with thymocytes isolated from immature rats. Peak accumulation of both total and specifically bound [3H]TCDD was observed at 60 min post incubation. Incubation of cells with 10 nM TCDD resulted in significant increases in RNA polymerase activity and incorporation of [3H]uridine at 30 min, indicating increased RNA synthesis in response to TCDD. TCDD-induced stimulation of [3H]uridine incorporation was not significantly altered in the presence of cycloheximide, while this effect was abrogated in the presence of actinomycin D. Incubation of thymocytes with 10 nM TCDD also stimulated the activity of poly(A)polymerase, the enzyme catalyzing mRNA polyadenylation, at time points beyond 30 min. No significant increases in [35S] incorporation were observed in cells treated with 10 nM TCDD, although analysis of detergent and high salt extracted nuclear proteins by SDS-PAGE and coomassie blue staining revealed the increased abundance of at least two proteins with molecular masses of 52,000 and 42,000 Da, respectively. These studies reveal that thymocyte nuclei rapidly accumulated TCDD in vitro, leading to increased RNA synthesis, poly(A)polymerase activity and protein synthesis. These events correlate closely with the process of programmed cell death.