After a shift in temperature, the population kinetics of HeLa cells does not immediately reach a steady state characteristic of the new temperature. During the transition period the duration of each period of the life cycle (period before DNA synthesis, of DNA synthesis, after DNA synthesis, and the period of mitosis) increases toward its steady-state value. Of the four periods, the period before DNA synthesis reaches its steady-state value fastest. Exponential growth can be maintained only from 33°C through 40°C. The period of mitosis is the most temperature-sensitive period of the life cycle of the cell. At subnormal temperatures (26°C through 31°C) there is an accumulation of cells in mitosis, and mitotic indices as high as 0.44 can be obtained. The duration of mitosis is a function both of the temperature and of the time which the cell has spent at this temperature.