This investigation is concerned with the internal factors governing the life span of individual yeast cells. The life span may be limited either by the number of buds a cell can produce or by internal measurement of metabolic time. The natural relationship between the number of cells a single cell can produce and the passage of time was modified by three different kinds of treatment: (1) by cooling the cells for several hours each day; (2) by preculturing the cells in media which inhibit cell division before allowing logarithmic growth; and (3) by culturing the cells in a medium which reduces the rate of budding. All these methods led to a prolongation of chronological life span, but the life span measured by the number of buddings remained remarkably constant. We therefore conclude that there is some kind of factor involved in the budding process which determines life span.