Cultures of Tetrahymena pyriformis, T. hegewishi and T. malaccensis have been studied with regard to control of cell volume and cellular dry weight. Cell volume was measured on cells suspended in 0.9% sodium chloride + 0.1% sodium azide using a Multisizer cell counter (Coulter). Tetrahymena were grown at different temperatures and under various up- or downshift conditions. In all cases the changes in cell volume are paralleled by changes in cellular dry weight. The volume and the dry weight of a Tetrahymena cell are determined by the particular medium and the growth temperature. Large cells are seen in concentrated media and at low growth temperatures resulting in cell volumes up to 17,000 microns 3, whereas starving cells decrease gradually towards 700 microns 3 or even smaller sizes. It is proposed that lag phase observed at up- and downshift is to a large extent due to the necessary adjustment of the cell volume to the new conditions.