The response of neurons to osmolal concentration changes has not been well-documented compared to erythrocytes, urinary bladder and epithelial cells. The effects of a hypotonic solution on morphology and electrophysiological functions in cultured dissociated neurons can be precisely studied. From the analysis of video pictures from Nomarski optics, diameters of the cells were seen to increase and then recover to the initial values after the application of a hypotonic solution. The rate of increase of cell size in fetal neurons was 4-5 times faster than in mature neurons. This age-related transient response was accompanied by a change of resting potential and membrane resistance. This transient depolarization and decrease of the resistance corresponded to morphological changes. However, the amplitude of an action potential scarcely changed during the cell membrane expansion. It is plausible that increased membrane tension caused by the membrane expansion might facilitate the opening of the channel. The cell membrane expansion might also increase ionic permeability through the membrane. This may reduce a swollen cell volume to the initial one by diluting their intracellular solute concentration. After this adaptation the cultured neurons were able to survive for a long time and extend processes in the hypotonic environment.