Morphologically, apoptosis is characterized by chromatin condensation, decrease in nuclear volume, and fragmentation of the nucleus. In the present study, we evaluated ultrastructural changes after UV-irradiation, using the microglial cell line, BV-2, as a model. In all stages of apoptosis that were examined (1, 3, and 5h after UV-irradiation), we identified condensed ribonucleoproteins in central areas of the nucleus. At early stages of apoptosis, chromatin gradually accumulated at the periphery of the nucleus and the nuclear volume decreased. In late stages of apoptosis, most of the nucleus was filled with condensed chromatin. Surprisingly, condensed chromatin was not only identified in the nucleus, but also in the cytoplasm. Furthermore, we found that accumulation and release of condensed chromatin correlated with dilated areas of the nuclear envelope. Although some of these dilated parts of the nuclear membrane pinched off from the nuclear envelope and were identified in the cytoplasm as vesicles, they did not contain condensed chromatin. Thus, UV-irradiation triggers nuclear degradation in BV-2 cells. However, in contrast to classical nuclear fragmentation, chromatin is translocated from the nucleoplasm to the cytoplasm by an unknown mechanism.