In view of the neurotoxic effects of mercuric chloride the following study was designed to explore its effects on brain serotonin and dopamine metabolism. Mercuric chloride injected at a dose of 1 mg/ml/kg increased locomotor activity, decreased latency to move, corner sittings and food intake. A decrease of 33% of immune response was observed in mercuric chloride treated rats as evident from anti nuclear antibody (ANA) positive test. Neurochemical analysis revealed that mercuric chloride administration increased brain tryptophan concentrations 5-Hydroxyindoleacetic acid (5-HIAA) and dihydroxyphenyl acetic acid, while 5-Hydroxytryptamine (5-HT) and Dopamine (DA) were decreased. The results also showed an increase in brain 5-HT and DA turnover rate in mercuric chloride injected rats. These increases in 5-HT and DA metabolism suggest that mercuric chloride may tend to contribute to anorexia and hyperactivity induced in drug administered rats.