In randomly grown tissue culture cells (mouse leukemic lymphoblast, L5178Y) the number, volume, and Na(+) and K(+) content increase as an exponential function with a doubling time of 11.3 hr. In synchronously grown cells the volume increase of the population and of single cells follows the same exponential function as in randomly grown cells. In contrast, the cation content fluctuates during a single cell cycle. About 1(1/2) hr after the cell division burst (at the beginning of the S period), a net loss of K(+) occurs for a period of about 1 hr amounting to about 20% of the total K. Over the next 5 to 6 hr, the deficit in K(+) is eliminated. The Na(+) content shows a double fluctuation. It falls during the cell division burst, rises when the K(+) content decreases, falls again when K(+) content rises, and then increases again before the next cell division burst. The net fluxes of both Na(+) and K(+) are very small compared to the unidirectional fluxes (less than 5%), thus small changes in the balance of influx and efflux account for the changes in cation content during the growth cycle. Both unidirectional fluxes increase dramatically (by a factor of two) about 2 hr after the cell division burst, and then remain constant until after the next cell division. The pattern of electrolyte regulation during cell division does not follow a simple function such as cell number, cell surface, or cell volume, but must be related to specific internal events in the cell.