Cell body volume and Alcian blue-Alcian Yellow staining properties of neurosecretory neurones in the brain of Lymnaea stagnalis were compared for snails kept in de-ionised water and standard tapwater. In the same experiment, the ionic content of the blood, blood volume and body weight and environmental ionic composition were measured. Five days of immersion in de-ionised water resulted in significant decreases in body weight, blood volume and blood, Na+ and Cl- concentrations but no change in blood Ca2+, K+,and HCO3- concentrations, compared with controls. No consistently significant differences across the 5 day period were found in cell body volumes for Dark Green Cells, Yellow Cells or Light Green Cells (used as a control) when these volumes were compared for large numbers of cells from snails kept in de-ionised water and standard tapwater. However, the number of Yellow Cells which could be counted in snails kept in de-ionised water was lower than the number from standard tapwater by day 2 of the experiment and lower for Yellow-green Cells by day 5. We interpret this lower number to be the result of depletion of Alcian blue-Alcian Yellow stained neurosecretory material in these cells which made them impossible to distinguish. This was confirmed by examination of visceral Yellow Cells which could be identified on the basis of known location close to the visceral-right parietal connective.