Cytokines are now considered as constitutive factors of the brain. Some of them are involved in the mechanism regulating lineage commitment and cellular differentiation of the central nervous system (CNS). We describe here the analysis of gene expression in cortex and hippocampus, of interleukin-1 alpha (IL1), interleukin-2 (IL2), interleukin-6 (IL6), macrophage-colony stimulating factor-1 (MCSF) and monocyte chemoattractant protein-1 (MCP1) in fetal (day 18 of gestation; G18), newborn (postnatal day 2; P2), young (postnatal day 21; P21) and adult rat using the reverse transcriptase-polymerase chain reaction (RT-PCR). IL6 and MCP1 mRNA presented distinct patterns of expression levels: IL6 mRNA level is most highly expressed in the embryonic cortex, whereas MCP1 is expressed at a maximal level in the postnatal day 2 cortical area. In the hippocampus, IL6 is most expressed at the adult stage and MCP1 exhibits an equal level of expression from day two to the adult stage. However, under our experimental conditions, IL1 alpha, IL2 and MCSF mRNA were not observed. Thus, certain cytokine genes, each with a specific pattern, are expressed in the rat CNS in adult and during ontogenesis. These observations suggest that cytokines might be involved as regulating factors promoting CNS development.