The distribution of various morphological forms of microglia stained with lectin in different regions of postnatal rat brain was examined in 2 planes of section, coronal and sagittal. In the coronal sections (mediolateral plane) taken at the level of the optic chiasma, the lectin-labelled cells were examined in the gustatory (GU), lateral somatosensory (LSS), medial somatosensory (MSS), motor (MO) and cingulate (CI) cortex. In the sagittal sections (rostrocaudal plane), the lectin-labelled cells in the medial orbital (MOA), motor (MO) and primary visual area (PVA) were studied. The cells in the motor area in coronal plane were further analysed with reference to their distribution in each of the laminae (layers). Based on the variation of their external morphology which represents different degrees of differentiation, all lectin-labelled microglial cells of the above-mentioned regions in newborn (P0) and rats aged 4 d (P4) and 8 d (P8) postnatum were classified and quantified. In the mediolateral plane of any given age group, the most differentiated ramified cells were located in GU except in P8 rats where the cells were also concentrated in CI. Of the 3 regions in the rostrocaudal plane the majority of the more differentiated ramified cells at P0 were found in the PVA but were the major cell type in MO in P4 and P8 rats. For the distribution of cells in MO, the most differentiated cells were located in the intermediate zones. It was concluded from this study that microglial cells in the developing cerebrum showed a gradient of differentiation in relation to different regions of the cerebral cortex but this appeared to vary with age. Thus in the mediolateral plane in P0 and P4 rats, the gradient extended from GU to CI in a lateromedial fashion but in P8, in the direction towards MSS from GU and CI. In the sagittal section, the gradient was directed caudorostrally in P0 rats. In P4 and P8 rats, however, the gradient was from MO to both poles (MOA, PVA). In the motor cortex, the gradient was from the intermediate zone towards the superficial and deep laminae. The gradient of differentiation of microglia may be related to the growth of the respective regions in the cerebral hemisphere but inherent genetic factors were also considered.