Serotonin functions as a neurotransmitter in the central nervous system and takes part in vascular tone, gastrointestinal motility, and blood coagulation at the periphery. New data on a correlation between serotonin level in platelets and cerebrospinal fluid (Audhya et al., 2012) have renewed interest in the hypothesis that considers a platelet as a model of serotoninergic neuron. In this study, using high performance liquid chromatography, we compared the serotonin level in platelets, serum and different brain regions in 6- and 24-month-old rats. It was found that serotonin level decreased from 0.768 to 0.359 μg per 109 cells in platelets and increased in midbrain from 0.260 to 0.439 μg per 1 g of wet weight during the animal aging. The differences between young and old animals in the serotonin level in serum and other brain regions were statistically not significant. Hence, despite the attractiveness of the hypothesis considering the platelet as a neuron model the data on the platelet serotonin transport should be extrapolated on the neuronal transport with caution, especially for the aging process.