Seminiferous epithelial cells of mouse testis were treated with hematoporphyrin derivative (HPD)-light in vitro. After HPD-light treatment, the changes within cells in early stage of prophase were observed. Examination with EM showed that 30 min after HPD-light treatment, the mitochondria were aggregated and vesiculated, the nucleus showed an irregular outline and a sign of chromatolysis; the other organelles showed no significant changes at this time. The results suggest that both mitochondria and nucleus are susceptible to HPD-light treatment. Three hours after HPD-light treatment, many of multi-nucleated spermatids appeared and they all contained deformed nuclei. This fact suggests that cytokinesis was inhibited by this treatment, and the chromatin of metaphase were also damage.