Previous studies have shown that initiation of proliferation of density-inhibited fibroblasts by fresh serum is accompanied by a rapid increase in phosphate uptake. This increase might be a key event in the initiation of DNA synthesis. The present studies examined this possibility. Mouse 3T3, secondary chick embryo, or human diploid foreskin cultures were grown to quiescence in medium containing varying levels of serum. When proliferation of the cultures was initiated by addition of fresh serum, the changes in phosphate uptake were inversely related to the final increases in cell number. Additional experiments showed that the change in phosphate uptake following serum addition was determined by the level of phosphate uptake prior to serum addition. Addition of dexamethasone to quiescent 3T3 cultures caused them to proliferate but did not increase phosphate uptake. Similarly, trypsin or insulin stimulated proliferation of quiescent secondary chick embryo cultures, but caused little or no change in phosphate uptake. Quiescent 3T3 cultures switched to medium containing fresh serum and reduced levels of phosphate showed a decrease in both phosphate uptake and intracellular phosphate pool size. Cell proliferation in these cultures, however, was stimulated to the same degree as cultures switched to medium containing fresh serum and the normal amount of phosphate. In addition, quiescent secondary chick embryo cultures switched to medium containing fresh serum and no phosphate showed a decrease in the intracellular phosphate pool size. Thymidine incorporation and final cell number in these cultures, however, was stimulated to the same or higher degree than in cultures switched to medium containing fresh serum and the normal amount of phosphate. These results demonstrate that the rapid increase in phosphate uptake following addition of fresh serum to quiescent fibroblasts is not a necessary event for the initiation of proliferation.