Exposure to AgNPs compromises neuronal viability and degenerates neuronal processes in well-established cells and network. To assess whether AgNPs could cause neurotoxicity of cells that had established neurite processes during an early stage of development, rat cortical cultures were first maintained in control culture medium for 4 days and allowed to develop processes and overlaps. Cultures were subsequently exposed to culture medium containing AgNPs at 0 (control-A), 1 (B), 5 (C), and 10 (D) μg/ml for 2 days. Phase contrast pictures were taken on day 6. To assess the consequence of AgNP exposure at a later stage of development, rat cortical cultures were first maintained in control culture medium for 10 days and allowed to develop a mature, well-endowed network; cells were subsequently exposed to culture medium containing AgNPs at 0 (control-E), 1 (F), 5 (G), and 10 (H) μg/ml for another 2 days. Phase contrast pictures were taken on day 12. Under both culture conditions, AgNPs at high concentrations (> 5 μg/ml) compromised cellular membrane integrity and induced neurite degeneration. It is however interesting to note that AgNPs induced fragmentation of neurites at the early stage of cell culture (C-D), while triggered an aggregation of cells bodies and thinning of neurite processes in the later stage of cell culture (G-H). In addition, glial cell layers (indicated by white circles) were present under the control (E) and 1 μg/ml (F) of AgNP- treated cultures, but were absent in cultures that were exposed to AgNPs at the concentrations of 5 (G) and 10 (H) μg/ml. Scale bar, 25 μm.