The spatial position of astrocytes between neurons and the vascular endothelium might allow the astrocytic syncytium to act as a pathway for diffusion or active transport of substances between the environment of neurons and the cerebral blood vessels and ventricles. Furthermore, the intimate morphological arrangements between astrocytes and neurons might reflect biochemical cooperations between the cells. Astrocytes in the adult nervous system are in an unique position to regulate the environment of neurons and thereby their sensitivity. Some questions are raised: How do astrocytes receive neuronal signals? Are there local populations of astrocytes which respond specifically to small groups of neurons? Late results suggesting astrocytes to be a candidate for the supervision of neurotransmission are commented upon.