Abstract The effect of platelet-derived growth factor (PDGF) on cellular Ca 2+ was examined in BALB/c-3T3 cells. PDGF induced: 1. 1. A decrease in cell 45Ca 2+ content. 2. 2. An apparent increased rate of efflux of preloaded 45Ca 2+. 3. 3. A decrease in residual intracellular 45Ca 2+ remaining after rapid efflux. 4. 4. When added after the rapid phase of efflux of 45Ca 2+ had occurred, an immediate decrease in post-efflux residual intracellular 45Ca 2+. All of the observed changes in 45Ca 2+ induced by PDGF are consistent with a rapid release of Ca 2+ from an intracellular Ca 2+ pool that has the slowest efflux and is relatively inaccessible to extracellular EDTA. When incubated with chlortetracycline (CTC), a fluorescent Ca 2+ probe, 3T3 cell mitochondria became intensely fluorescent. Addition of PDGF resulted in a rapid decrease in CTC fluorescence intensity in both adherent and suspended 3T3 cells. The effects of PDGF on 3T3 cell Ca 2+ stores and CTC fluorescence intensity were identical with the effects of the Ca 2+ ionophore A23187 and of the proton ionophore carbonyl cyanide m-chlorophenyl hydrazone. Serum, which contains PDGF, also altered intracellular Ca 2+ stores, but platelet-poor plasma, which does not contain PDGF, had no effect. EGF, insulin, and tetradecanoyl phorbol acetate (TPA), other factors which stimulate 3T3 cell growth, did not alter 3T3 cell Ca 2+ stores. Release of Ca 2+ from intracellular sequestration sites may be a mechanism by which PDGF Stimulates Cell growth.