The effect of administration of aluminum to rats on the level of three phospholipase C (PLC) isozymes (beta 1, gamma 1, and delta 1) was assessed in a variety of brain tissues. After exposure to aluminum, a statistically significant increase in malondialdehyde, an index of lipid peroxidation, was observed. In addition, there was a significant reduction in the catalytic activity of low molecular weight phosphotyrosine phosphatase, which loses its activity during oxidative stress. This suggests that oxidative stress is induced in brain tissues exposed to aluminum. The protein level of PLC-delta 1, but not that of PLC-beta 1 or -gamma 1, was significantly increased in brains where oxidative stress had been induced. The total PLC activity in aluminum-treated rat brains was significantly higher than that in control brains. These results suggest that PLC-delta 1 protein levels in brain tissues are increased by the induction of oxidative stress, giving an explanation for its up-regulation in Alzheimer's disease.