The present study investigated the effects of serotonergic dysfunction on working memory. Therefore, the effects of inhibition of serotonin [5-hydroxytryptamine (5-HT)] synthesis induced by p-chlorophenylalanine (p-CPA) and pharmacological blockade of 5-HT receptors by methysergide on the performance of rats in a delayed nonmatching to position task assessing spatial working memory were studied. Methysergide (1.0, 5.0, or 15.0 mg/kg) significantly disrupted behavioral activity of rats and decreased the percent correct total responses. However, the impairment in the percent correct responses was delay independent, indicating a nonmnemonic disruption of the performance. p-CPA (500 mg/kg/day x 3) induced an almost total depletion (> 97%) of frontal cortical and hippocampal serotonin and its major metabolite 5-hydroxyindoleacetic acid and slightly affected noradrenergic and dopaminergic systems. p-CPA treatment did not affect the percent correct responses. However, the behavioral activity of rats was slightly decreased by p-CPA. The disruptions in behavioral activity and the percent correct responses induced by methysergide (2.0 mg/kg) were not abolished by p-CPA. The present results do not support any important role for the serotonergic system in spatial working memory as assessed using the delayed nonmatching to position task.