In the guard cells of Allium cepa leaves, no starch was found either when the stomata were open or closed. The lack of other soluble polysaccharides that could be hydrolyzed during the opening reaction of the stomata (Schnabl, Planta 1977, in press) leads to the question, how is the osmotic effect, which is the basis of the stomatal movement, achieved in Allium? It is shown in this paper, by histochemical and microprobe analyses, that in Allium - as in other plant species-the K(+) concentration of the guard cells increases during stomatal opening. The charges of the K(+) ions in the guard cells seem to be fully compensated by imported Cl(-) ions. This could mean that if starch is present in the guard cells, as in the majority of plant species, its major role in the mechanism of stomatal movement is to deliver the cuunteranions for the imported K(+) ions.