Experimental infection of rhesus monkeys (Macaca mulatta) with a virulent (W1) strain of Plasmodium knowlesi resulted in cerebral malaria. Electron microscopical examination of the brain revealed large numbers of intracytoplasmic lipofuscin pigment deposits in cerebellar Purkinje neurones and choroid plexus epithelium of the lateral ventricle. This lesion may be part of the nervous system response to ischaemic hypoxia.