This thesis addresses: 1) The involvement of the monoaminergic and neuropeptidergic neurotransmitter systems in the aging process, and 2) The contribution of these systems in the etiology of an age-related mental disorder: the depressive syndrome. Because of the evidence suggesting the efficacy of the SSRI in the treatment of some age-related symptoms, a comparison study of the binding characteristics of two SSRI, i.e. [3H]paroxetine and [3H]citalopram, to the human brain 5-HT uptake site was included.Neurochemical analysis, involving HPLC, binding assays and RIA methods, was performed in several brain regions from 23 control subjects and 18 suicides pooled according to the method of death and the prior existence of depressive symptoms.Both [3H]paroxetine and [3H]citalopram were found to label the same number of presynaptic 5-HT binding sites, which is in accordance to their ability to identify the same membrane protein. However, the 15 to 30-fold higher affinity displayed by [3H]paroxetine gives evidence to this SSRI having a more easily accesible binding domain in the 5-HT transport complex than [3H]citalopram, and hence being a better marker of thispresynaptic 5-HT carrier system.No statistical differences in either the monoamines 5-HT, NA and DA, their metabolites or the 5-HT binding sites were found between controls and overall suicides. However, a diminished number of 5-HTl D binding sites with advancing age, together with a significant decrease in the number of 5-HTl D binding sites and binding affinity was noticed in the nondepressed and depressed suicides, respectively. These results might indicate the involvement of this novel 5-HT receptor in both some of the physical disturbances present in the elderly population and in the mechanisms underlying the depressive syndrome. Advancing age was also found to be negatively correlated with brain NA, DA and HV A concentrations, thus supporting the increased likelihood of changes in feeding habits and in the hypothalamic-mediated endocrine dysfunctionscommonly observed in senescence.With regard to the neuropeptidergic neurotransmitter systems, age-related decreases in gyrus cinguli NPY and CRF concentrations were noted. In addition, although unchanged NPY, SOM and CRF concentrations were observed in the overall suicide group, the HPLC analysis revealed that the depressed suicides showed a different pattern of NPY-LI fragments, which is in agreement with depression being associated with an altered processing or metabolism of the intact NPY molecule.