After a 5-min heating of isolated Tradescantia leaves at 49 degrees C the granular component in the nucleoli of parenchyma cells disappears and the nucleoli grow more compact and exclusively fibrillar in structure. Simultaneously, the ability of chloroplasts of these cells for phototaxis is completely inhibited. In leaves placed in a moist chamber at room temperature 48 h after heating a normal ultrastructure of nucleoli and the ability of chloroplasts to respond by phototaxis to change in illuminations are restored almost synchronously.