The beta-Amyloid peptide (A beta) is hypothesized to mediate the neurodegeneration seen in Alzheimer's disease. Recently, we proposed a new hypothesis to explain the toxicity of A beta based on the free-radical generating capacity of A beta. We have recently demonstrated using electron paramagnetic resonance (EPR) spectroscopy that A beta (1-40) generates free radicals in solution. It was therefore suggested that A beta radicals can attack cell membranes, initiate lipoperoxidation, damage membrane proteins, and compromise ion homeostasis resulting in neurodegeneration. To evaluate this hypothesis, the ability of A beta to induce neuronal oxidation, changes in calcium levels, enzyme inactivation, and neuronal death were compared with the ability of A beta to produce free-radicals. Using hippocampal neurons in culture, several methods for detection of oxidation were utilized such as the conversion of 2,7-dichlorofluorescin to 2,7-dichlorofluorescein, and a new fluorescence microscopic method for the detection of carbonyls. The ability of A beta to produce free-radicals was determined using EPR with the spin-trapping compound N-tert-butyl-alpha-phenylnitrone. Consistent with previous studies, we found that preincubation of A beta increased the toxicity of the peptide. There is a strong correlation between the intensity of radical generation by A beta and neurotoxicity. The highest neuronal oxidation and toxicity was seen at a time when A beta was capable of generating the most intense radical signal. Furthermore, little oxidation and toxicity was seen when cultures were treated with freshly dissolved A beta, which did not generate a detectable radical signal. These data are consistent with the hypothesis that free-radical-based oxidative damage induced by A beta contributes to the neurodegeneration of Alzheimer's disease.