Malathion is a highly neurotoxic pesticide widely used in daily life. Acute and chronic toxicity from this organophosphorus compound may cause damage to health, especially to the central nervous system. In the present work, we show the effects of chronic exposure of malathion on dendritic morphology of neurons from prefrontal cortex (PFC), hippocampus, and nucleus accumbens (NAcc) in adult male mice. Animals were injected i.p. with low dose of malathion (40 mg/kg body weight) for 14 days. Control animals were injected with corn oil, used as vehicle. Fourteen days after the last injection, brains were removed and processed by the Golgi-Cox stain method, and coronal sections were obtained to perform Sholl analysis on pyramidal neurons from the PFC, CA1 area from the hippocampus, and medium spiny cells from the NAcc. Dendritic morphology analysis included the total dendritic length, the maximum branching order, and the dendritic spine density. Results indicated a significant decrement on dendritic morphology in neurons from the hippocampus and the PFC in animals injected with malathion, whereas medium spiny neurons from NAcc showed a significant decrement only on the dendritic spine density in malathion injected mice, as compared to control mice. These results suggest that chronic toxicity of malathion alters the dendritic morphology in adult age, which may affect behavior.