The present study was designed to investigate the neuropathological effect of mercuric chloride in relation to physiological response on terrestrial slug Semperula maculata using topical application technique (contact action). Lethal concentration (predetermined LC50 value-107 ppm) of HgCl2 was used and experiment lasted for 4 days. Histopathological and ultrastructural alterations in the cerebral ganglia were more obvious after exposure of HgCl2 by topical application technique. The alteration includes shrinkage of perikarya of neurons, increased cytoplasmic basophilia and extreme indentation of the plasma membrane. In addition, the multiple karyosomes, eccentric and irregular nuclear envelope and its effect on physiological responses were recorded in the study.