Primary neuronal culture is a powerful tool to study neuronal development, aging, and degeneration. However, cultured neurons show signs of cell death after 2 or 3 weeks. Although the mechanism underlying this phenomenon has not been elucidated, several preventive methods have been identified. Here we show that the neuronal loss in primary cortical culture involves calpain activation and subsequent neuronal cell death. Neuronal loss during cultivation showed destruction of neurites and synapses, and a decrease in neuron numbers. mu-Calpain and m-calpain were initially activated and accumulated by increased RNA expression. This neuronal death exhibited neurodegenerative features, such as conversion of p35 to p25, which is important in the developmental process and in the pathogenesis of Alzheimer's disease. But, postnatal and aged rat cortex did not show calpain activation and prolonged processing of p35 to p25, in contrast to the long-term culture of cortical neurons. In addition, the inhibition of calpains by ALLM or ALLN blocked the conversion of p35 to p25, indicating that the calpain activity is essential for the neurodegenerative features of cell death. Taken together, this study shows that the neuronal loss in primary cortical cultures involves neurodegeneration-like cell death through the activation of calpains and the subsequent processing of p35 to p25, but not developmental apoptosis or aging. Our results suggest that the long term primary culture of cortical neurons represent a valuable model of neurodegeneration, such as Alzheimer's disease.