Serotonin functions as neurotransmitter in central nervous system and is involved in the regulation of vascular tone, gastro-intestinal motility and blood coagulation in the periphery. The appearance of new data on the significant correlation between serotonin levels in platelets and cerebrospinal fluid (Audhya et al., 2012) renewed interest in the hypothesis in which the platelet is seen as a model of cerotoninergic neuron. In our study, the levels of serotonin in platelets, serum and various brain regions of rats aged 6 and 24 months have been determined and comparatively analyzed. The method of high performance liquid chromatography was used. The decrease of serotonin level in platelets from 0.768 to 0.359 μg per 10(9) cells and its increase in the middle brain from 0.260 to 0.439 μg per 1 of wet weight have been clearly demonstrated in aging of animals. The differences in the content of serotonin in other parts of the brain and in the blood serum of young and old animals were statistically insignificant. Therefore, despite the attractiveness of the concept of platelet as a model of a neuron, the extrapolation of the data on platelet serotonin transport into neuronal ones requires caution, especially in the study of aging.