Pinocytotic activity was investigated in rat cerebral cortex using the soluble tracers horseradish peroxidase and Lucifer yellow. A subpopulation of cells selectively accumulated both compounds and the labelling was mainly present in pinocytotic vesicles associated with the cell body. Labelled cell bodies were small, round to oval, and distributed in an almost regular array throughout the tissue. Based on distinctive morphological features, some of the labelled cells could be determined as ramified microglia. This identification was confirmed by immunofluorescence staining with the monoclonal antibody OX-42, which specifically recognizes microglia; OX-42 staining consistently co-localized with pinocytotic labelling. The possibility that ramified microglial cells perform a normal function of continuous fluid exchange in brain tissue is discussed.