Ultrastructural changes of the central nervous system (CNS) were compared in inbred White Leghorn chickens that are genetically resistant or susceptible to Marek's disease virus (MDV)-induced transient paralysis (TP). Widened extracellular spaces, containing granular material resembling protein were found in brains of TP-affected chickens. Demyelination was absent in all chickens and intramyelinic oedema was rare, present only in TP-affected chickens. Both MDV-inoculated TP-resistant and TP-susceptible chickens had CNS perivascular mononuclear cell cuffs at all sampling periods, but the TP-affected chickens also had endothelial cell hypertrophy, a greater quantity of intramural phagocytized chromatin debris, and intramural heterophils. Such findings suggest vasculitis as the causative lesion. No open endothelial cell tight junctions, endothelial cells discontinuities, or transendothelial channels were morphologically evident to explain the vasogenic oedema, but an apparent suggestive increase in coated micropinocytotic vesicles in CNS endothelial cells is a possible mechanism to explain the altered blood-brain-barrier.