Rates of uptake of several labeled compounds were measured during the cell cycle for three strains of Escherichia coli in balanced growth. Uptake rates were constant during more than the first two-thirds of the cycle, or reasonably so, for all of these compounds: glycine, leucine, glucose, acetate, phosphate, sulfate, and thymidine. When added de novo, uptake of glycine and leucine were not constant, but appeared to be proportional to mean cell volume. These results are in agreement with the finding that cell sizes increase linearly during most of the cell cycle for E. coli. They support the hypothesis, for cultures in balanced growth, that linear growth during the cell cycle is due to constant rates of uptake of all major growth factors. They also support the interpretation that uptake is limited by the presence of a constant number of functional binding or accumulation sites for these growth factors.