Calcium influx through glutamate receptors and voltage-dependent channels mediates an array of functional and structural responses in neurons. However, unrestrained Ca²⁺ influx can injure and kill neurons; a mechanism implicated in both acute and chronic neurodegenerative disorders. Data reported here indicate that depolymerization of actin filaments can stabilize intracellular free calcium levels ([Ca²⁺]_i) and protect hippocampal neurons against excitotoxic injury. Studies with fluorescein-labeled phalloidin showed that cytochalasin D and glutamate each induced actin filament depolymerization. The microfilament-disrupting agent cytochalasin D protected cultured rat hippocampal neurons against glutamate toxicity, whereas the actin filament-stabilizing agent jasplakinolide potentiated glutamate toxicity. The microtubule-disrupting agent colchicine was ineffective in protecting neurons against glutamate toxicity. Cytochalasin D did not protect neurons against calcium ionophore toxicity or iron toxicity, indicating that its actions were not due to nonspecific effects on Ca²⁺ or free radical metabolism. Cytochalasin D markedly attenuated kainate-induced damage to hippocampus of adult rats, suggesting an excitoprotective role for actin depolymerization in vivo. Elevations of [Ca²⁺]_i induced by glutamate were attenuated in cultured hippocampal neurons pretreated with cytochalasin D and potentiated in neurons pretreated with jasplakinolide. The [Ca²⁺]_i response to a Ca²⁺ ionophore was unaffected by cytochalasin D, suggesting that actin depolymerization reduced Ca²⁺ influx through membrane channels. Taken together with previous patch clamp data, our findings suggest that depolymerization of actin in response to Ca²⁺ influx may serve as a feedback mechanism to attenuate potentially toxic levels of Ca²⁺ influx.