Catecholamines (CA) are among the most well known neurotransmitters (NT) which act in a wide range of brain structures and are involved in many important functions, such as general arousal, autonomic, neuroendocrine and motor control and possible in emotion and mentation. The neuropharmacology of the brain CA synapses has played a crucial role in understanding the complex phenomena of the central neurotransmission and the feed-back mechanisms by which the central neurones adapt optimally to the functional needs at any time. The model of neurotransmission, as well as the mechanism by which various substances act at the synaptic level are briefly outlined. Due to the fact that the CA are implicated in many important functions of the brain, it has been suggested that certain mental diseases such as depression, as well as mental and behavioral manifestations of the ageing brain might have their origin in an impairment and particularly in a functional deficiency of the CA occurring at various structures. The available evidence in support to this view is outlined and the hypothesis that senile mental deterioration and the mental impairment encountered in various types of dementia (senile, presenile and multi-infarct) may be due to deficiency of the brain CA is discussed. Since the CA and particularly the dopamine deficiency seems to be a common denominator in depression and in senile mental deterioration, it is suggested that the dopaminergic drugs may serve as useful therapeutic means on one hand and important tools for testing the validity of the above hypotheses on the other.