Neuronal death was produced in the CA1 and CA3 areas of the hippocampus, amygdala, and piriform and entorhinal cortices after intraperitioneal administration of kainic acid at convulsant doses to adult rats. To assess the involvement of members of the Bcl-2 family in cell death or survival, immunohistochemistry, western and northern blotting to Bcl-2, Bcl-x and Bax, and in situ hybridization to Bax were examined at different time-points after kainic acid treatment. Members of the Bcl-2 family were expressed in the cytoplasm of pyramidal neurons in the hippocampus, and in a subset of neurons of the piriform and the entorhinal cortices, amygdala and neocortex in the normal adult brain. Dying neurons in the pyramidal cell layer of CA1 and CA3 areas, entorhinal and piriform cortices, and amygdala also expressed Bcl-2, Bax and Bcl-x following excitotoxicity, although many dying cells did not. In addition, a number of cells in the affected areas showed Bax immunoreactivity in their nuclei at 24-48 h following kainic acid administration, thus indicating Bax nuclear translocation in a subset of dying cells. Western blots disclosed no modifications in the intensity of the bands corresponding to Bcl-2, Bcl-x and Bax, between control and kainic acid-treated rats. No modifications in the intensity of the bcl-2 messenger RNA band on northern blots was observed in kainic acid-treated rats. However, a progressive increase in the intensity of the bax messenger RNA band was found in kainic acid-treated rats at 6 h, 12 h and 24 h following kainic acid administration. Interestingly, a slight increase in Bax immunoreactivity was observed in the cytoplasm of neurons of the dentate gyrus at 24-48 h, a feature which matches the increase of bax messenger RNA in the same area, as shown by in situ hybridization at 12-24 h following kainic acid injection. The present results suggest that cell death or survival does not correlate with modifications of Bcl-2, Bax and Bcl-x protein, and messenger RNA expression, but rather that kainic acid excitotoxicity is associated with Bax translocation to the nucleus in a subset of dying cells.