The nucleoli of tissue discs cut from stem tubers of Helianthus tuberosus L. or roots of Daucus carota L. increase in size when the discs are shaken in aerated distilled water. The size increase of up to several hundred per cent which occurs within 24 h is followed by a significant decrease in size within the next 24 h. Addition of 10-5M gibberellic acid to the system accelerates the beginning of the size increase and finally results in a larger nucleolus.