The brain-derived neurotrophic factor (BDNF) has been involved in pre- and postnatal brain development. Moreover, abundant levels of this neurotrophin have been found in animal and human brain and serum. This study was aimed to assess the postnatal change profile of both serum and brain BDNF levels. By using immunoassay and reverse transcription-polymerase chain reaction methods, BDNF protein and mRNA levels were determined in serum and platelets, respectively, and in two brain structures (hippocampus and frontal cortex) of postnatal rats (one and three weeks old), young adults (two months old) and in aged animals (two years old). The results showed that brain and serum BDNF levels underwent similar changes during maturation and aging processes (analysis of variance (ANOVA): P<0.001, P<0.001, for hippocampus and serum, respectively). During the same investigation period, the measure of BDNF mRNA indicated gradual changes in the hippocampus but not in platelets (ANOVA: P<0.001 and not significant, for hippocampus and platelets, respectively). Interestingly, there was a positive correlation between serum and cortical BDNF levels (r=0.81, P<0.01), especially in young animals. This study of ontogenic characteristics of BDNF in blood and central nervous system can help to shed more light on the role of platelet BDNF.