To assess the effects of methylphenidate (MP) on cerebral blood flow (CBF), 5 healthy males were studied using 15O-water and positron emission tomography before and after MP (0.5 mg/kg iv). MP significantly decreased whole brain CBF at 5-10 minutes (25 +/- 11%) and at 30 minutes (20 +/- 10%) after its administration. Decrements in CBF were homogeneous throughout the brain (regional decrements 23-30%) and probably reflect the vasoactive properties of MP. The vasoactive properties of MP should be considered when prescribing this drug chronically and/or when giving it to subjects with cerebrovascular compromise.