The ultrastructural appearances of microglia in the rat cerebral cortex were examined following experimental cold lesions. It was found that microglia toop up extravascularized plasma by pinocytosis. These pinocytotic vesicles coalesced to form large vacuoles of medium electron density, which subsequently decreased in size and increased in density and morphological complexity. Concurrently the microglial cells underwent dedifferentiation and division. Between 3 and 7 days after making the cold lesion it was difficult to distinguish between phagocytes of microglial and haematogenous origin. The relationship of microglia to haematogenous phagocytes is discussed.