The tissue of the CNS (central nervous system) is composed of neurons and neuroglia. Whereas neurons develop an ability for the rapid transduction of specific signals, astrocytes develop an ability to modulate the extracellular neuronal environment, and in mature CNS tissue manifest a capacity for active uptake of amino acids and ions. Astrocytes can control extracellular volume by regulation of their own volume, and are intimately involved in the neuronal exchange of trophic substances and metabolites. Astrocytic processes extend to blood vessel walls, the brain surface, the ventricular wall, neuronal cell bodies and synapses. Astrocytes are abundantly supplied with membrane receptors for various neurotransmitters, coupled to such second messenger systems as cyclic AMP (adenosine monophosphate) or the phosphatidylinositol cycle. Activation of the receptors results in changes in oxidative metabolism, cell morphology, cell volume, and immunocompetence: and recent findings have shown the occurrence of receptor-mediated changes in amino acid uptake. Thus, by modulating the extracellular environment, astrocytes can simultaneously modulate the sensitivity and/or excitability of large numbers of neurons. In the article are presented recent research findings suggesting astroglial cells to be targets for neurotransmitters, and probably to be actively involved in higher cognitive functions. Advances in our knowledge of astroglial cell characteristics might improve our understanding of behavioural disturbances and diseases of the CNS.