To study the mechanism of photodynamic nerve cell killing, isolated crayfish mechanoreceptor neurons were photosensitized by the sulfonated aluminum ophthalocyanine Photosens. Neuron activity was continuously recorded until irreversible abolition. Intense (10(-5) M Photosens) or weak (10(-7) M Photosens) photosensitization induced different bioelectric neuron responses: firing activation followed by irreversible depolarization block or gradual inhibition until firing abolition, respectively. These bioelectric responses were accompanied by different biochemical and morphological changes. In the case of intense photosensitization, neuron nuclei swelled and then shrank. Succinate dehydrogenase (SDH) was inhibited, and the plasma membrane was compromised just after firing cessation. Weak photosensitization did not induce these changes but caused swelling of the endoplasmic reticulum and destruction of the matrix, cristae and membranes in some of the mitochondria. Other mitochondria, however, retained the normal structure. Plasma membrane damage, SDH inhibition, nucleus shrinkage and impairment of the nuclear border occurred after 2-4 h. It is concluded that intense photosensitization induced necrotic processes during irradiation, whereas weaker impact caused delayed necrosis 2-4 h later. The observed electrophysiological neuron responses to photodynamic therapy may be considered as early hallmarks of different modes of forthcoming cell death.