The changes in cell nuclei in the stomata of Vicia faba were investigated using several different techniques. Illuminated stomata have rounded nuclei with homogenous chromatin, which is interpreted as functional structure. After darkening stomata contain extended nuclei with the regular chromonematic structure. It could be shown that after treatment with KCNS the nuclei are similar to that of the closed stomata and after treatment with KNO3 they are similar to that of the opened stomata.