Thymic programmed cell death (PCD) or apoptosis (Ao) is elevated during inflammation by a variety of stressors in vitro (i.e., glucocorticoids, tumor necrosis factor (TNF), prostanoids, etc.), however, little or no information is available concerning its presence in polymicrobial sepsis. To establish whether or not PCD is accelerated in the thymus following the onset of sepsis, thymocytes were harvested from C3H/HeN mice at 1, 4, 12, and 24 h following cecal ligation and puncture (CLP; to induce sepsis) or Sham-CLP (Sham), and assessed for changes in thymocyte viable cell yield, increased Ao + cells based on FACS analysis (propidium iodide staining) or by evidence of fragmentation of the genomic DNA. The results indicate that at 1 h post-CLP there were no marked changes in any of these parameters. However, by 4 h post-CLP the percentage of Ao + thymocytes increased and the septic mouse genomic DNA exhibited trace amounts of fragmentation. These changes increased in the septic animals cells through both 12 and 24 h. Alternatively, thymic viable cell yield did not significantly decrease until 12 h. Marked changes in systemic mediators, corticosterone and TNF, were also detected in septic mouse blood at all time points. In an effort to determine the contribution of these two agents to the induction of the accelerated PCD seen here, mice were randomized to receive either RU-38486 (11β-[ρ-(dimethylamino)phenyl]-17β-hydroxy-17-(1-propynyl)estra-4,9-dien-3-one (Mifepristone); a steroid receptor blocker), polyethylene glycol (PEG)-(rsTNF-R1)2 (a TNF inhibitor) immediately following CLP. The decline in viable thymocyte cell yield induced by sepsis (CLP) was not seen with RU-38486, but was present in PEG-(rsTNF-R1)2-treated mice. Furthermore, RU-38486, unlike PEG-(rsTNF-R1)2, treatment markedly decreased the percentage of cells which were Ao + as well as the extent of DNA fragmentation. Thus, sepsis-induced PCD in the thymus is not a response to TNF but appears to be controlled in vivo by corticosteriods released after the onset of sepsis.