The morphometric and quantitative changes associated with the differentiation of amoeboid microglia into ramified microglial cells in the corpus callosum of rats between 21 d postconception (E21) and 15 d postnatally are described. Using lectin labelling, 5 morphological types of labelled cells (R, SP, KLP, TLP, AP) based on cell body shape, the configuration of their cytoplasmic processes and their staining intensity, were recognised. Round cells (R) and cells with stout processes (SP) were aggregated in the central part of the developing corpus callosum whereas the highly branched labelled cells were distributed at its periphery. When the morphometric data and labelling intensities of labelled cells were analysed with the aid of an image analysis system, the values for cell length, area and perimeter increased as the complexity of branching increased, whereas the lectin-labelling intensity became reduced. Quantitative study showed that the proportion of the different morphological types of lectin-labelled cells peaked at different ages. The sequential peaking of R, SP and highly branched cells with advancing age suggests a similar chronological order of differentiation of R into branched cells. The quantitative study also showed a rapid increase in the density of lectin-labelled cells in the postnatal period between P4 and P8, attributed primarily to the active proliferation of the cell type. The consequent reduction of cell density (after P13) was probably due to cell death, a feature which appeared to increase with development.