The death of rat thymocytes after irradiation or hydrocortisone injection is accompanied by degradation of DNA. DNA degradation in the thymocytes of irradiated animals starts after a 2-hr lag, reaches its maximum by the 6th hour and remains constant between the 6th and 10th hours. The accumulation of chromatin fragments after hydrocortisone injection starts without a pronounced lag period and reaches its maximum (approximately equal to 15%): by the 9th-10th hour. The amount of degraded DNA is correlated with the number of cells with psychotic nuclei. The chromatin degradation products in rat thymocytes after irradiation or hydrocortisone injection are nucleosomes and their oligomers. The number of intranucleosomal breaks in DNA is negligibly small. In vivo experiments with inhibitors of RNA and protein synthesis in the system described have demonstrated that chromatin degradation depends on protein synthesis and is almost independent of RNA synthesis. Thus, the molecular mechanisms of nuclear DNA degradation in dying thymocytes appear to be similar and independent of the nature of the agents used.