Organophosphorus compounds (OPCs) are widely used as pesticides, but often show high toxicity in mammalian cells. To assess the toxic potential of OPCs, we examined the cytotoxicity of paraoxon, methyl-paraoxon, parathion, and methyl-parathion exposures on NB41A3 neuroblastoma mouse cell lines. The LC50s (median lethal concentrations) at 24 hr of exposure were determined from the acute toxicity test including time course experiments. The LC50 values suggest higher toxicity of paraoxon (0.42 mM) compared to parathion (0.66 mM). In addition, the methylated derivatives of both OPCs indicated similar but slightly lower levels of toxicity compared to paraoxon and parathion (0.46 mM for methyl-paraoxon and 0.77 mM for methyl-parathion). However, the results from time course experiments indicated obvious reduction of cell viability for parathion and methyl-parathion as early as 1 hr of exposure, whereas the effects of paraoxon and methyl-paraoxon were not significant before 6 hr of exposure. We also report the most affected biological processes in NB41A3 cells in response to parathion exposure using microarray experiment. Among the statistically overrepresented biological processes are the ones related to neuronal development, apoptosis, cell stress, and cell signaling.