Haloperidol, a neuroleptic drug, is still discussed if it is neurotoxic. In the present study, neurotoxic effects of haloperidol on guinea pig hippocampus were examined. Sixteen guinea pigs were randomly divided into two groups of 8 animals each. The animals of treatment group were given intraperitoneally (i.p.) haloperidol diluted in saline (dose of 5 mg/kg daily) for 6 weeks. Control animals received only the same volume of saline i.p. Hippocampi from both groups were examined by light and electron microscope. Numerous dark neurons with a heterochromatic nucleus and dense cytoplasm were observed in the hippocampi of the treatment group by light microscope and ultrastructural levels. Also some neurons have increased vacuolar contents and lysosomes. These findings suggest that haloperidol leads to prominent damage of hippocampal neurons.