Amyloid beta-peptide (A beta) is the principal component of neuritic plaques in the brain in Alzheimer's disease (AD). Recent studies revealed that A beta can be neurotoxic by a mechanism involving free radical production and loss of cellular ion homeostasis, thus implicating A beta as a key factor in the pathogenesis of AD. However, other proteins are present in plaques in AD, including the protease thrombin and protease nexin-1 (PN1), a thrombin inhibitor. We therefore tested the hypothesis that thrombin and PN1 modify neuronal vulnerability to A beta toxicity. In dissociated rat hippocampal cell cultures the toxicity of A beta was significantly enhanced by coincubation with thrombin, whereas PN1 protected neurons against A beta toxicity. A beta induced an increase in levels of intracellular peroxides and calcium. Thrombin enhanced, and PN1 attenuated, the accumulation of peroxides and calcium induced by A beta. Taken together, these data demonstrate that thrombin and PN1 have opposing effects on neuronal vulnerability to A beta and suggest that thrombin and PN1 play roles in the pathogenesis of neuronal injury in AD.