Prepulse inhibition (PPI) of startle response is a valuable paradigm for sensorimotor gating processes by which an organism filters sensory information. Disruption of PPI is evident in schizophrenia and Huntington's disease among other neuropsychiatric disorders characterized by sensorimotor gating deficit. In such disorders, increased brain monoamines' transmission was suggested to underlie PPI disruption. The effect of St. John's wort extract on PPI of startle response is yet to be investigated despite research findings indicating augmented levels of brain monoaminergic function after acute administration of the extract. In this study, the effect of acute oral administration of St. John's wort extract (62.5, 125, 250 and 500 mg/kg) was tested on PPI of an acoustic startle response in rats. A disruption of PPI resulted after the administration of the highest dose of the extract and therefore its effect on dopaminergic, serotonergic and noradrenergic neurotransmission was investigated in a number of rat brain areas involved in mediation of PPI response. Results demonstrated that 500 mg/kg of the extract augmented dopaminergic transmission in the thalamus together with elevating noradrenergic transmission in both brainstem and hippocampus. In addition, increased serotonin levels in brainstem, thalamus, cortex and hippocampus were evident after administration of the same dose of the extract. It was therefore concluded that St. John's wort extract (500 mg/kg) reduced PPI response possibly through enhancing monoaminergic transmission in brainstem, thalamus, cortex and/or hippocampus. Clinical investigations are therefore needed to determine whether St. John's wort extract may affect sensorimotor gating in both schizophrenia and Huntington's disease.