The mitotic index of suspensions of Ehrlich Ascites Tumor Cells in non-nutrient media decreased exponentially by a factor of 10 during 8 hours, whereas, in blocks of regenerating liver it decreased by a factor only 1.7 during 8 hours, and 4 in 24 hours. Proportions of cells in different mitotic phases remained constant throughout 24 hours in blocks of regenerating liver, whereas, in EAT Cells mitotic figures are limited to the prophase after 4 hours. These observations support the belief that cells having entered the prophase complete the cycle independent of outside energy sources, that extraneous energy is essential to entering the prophase, and that tissue fluids present in fresh tissue blocks provide nutrients adequate for maintenance of mitotic activity at the original level for about 4 hours prior to fixation.