Changes in the size and dry mass of glioma C-6 intact cells in culture were investigated for 35-50 min at 5 min intervals by means of vital cytointerferometry. Rhythmic variations in the size, dry mass and protein concentration were thereby revealed in glioma cells. These variations fall into the category of circahoralian ones. While considerable variations in the cell area and dry mass were observed, changes in protein concentration were less pronounced. Addition of dibutyryl cylic AMP (db-cAMP), in a concentration of 10(-3) mol/l to the cultivation medium, produced no effect on the rhythm of the above parameters in glioma C-6 cells.