Calcium and magnesium contents were measured in cells of Tetrahymena pyriformis induced to divide synchronously by a multi-heat-shock procedure. During free-running synchronized cell division in complex proteose peptone medium, significant peaks of both calcium and magnesium were observed at points in the cell cycle just prior to division. No such peaks were detected in cells dividing asynchronously in proteose peptone. When synchronized cell division was followed after transfer to an inorganic medium, cell calcium and magnesium levels were observed to decrease in relation to the corresponding cell number increase, indicating that in concentration terms, calcium and magnesium remain fairly constant. This latter result suggests that neither calcium nor magnesium influxes act as triggers for cell division in Tetrahymena and that the fluctuations of these metals seen during the synchronized division cycle in complex medium represent an effect rather than a cause.