ALTERATIONS in the permeability of the surface membrane might play a key role in the control of DNA synthesis and cell division1–3. Evidence that changes in hexose transport might participate in growth control comes from reports that uptake of certain hexoses (1) increases after transformation by tumour viruses4–6, and correlates with the expression of the transformed phenotype7; (2) decreases when untransformed fibroblasts reach a density-inhibited monolayer5,8, and (3) rapidly increases after initiating cell division with serum or other agents8,9. We now report, however, that increased hexose transport and initiation of proliferation can be uncoupled in density-inhibited 3T3 mouse fibroblasts. Levels of cortisol which initiate DNA synthesis and division of these cells10,11 actually decrease hexose transport. In addition, some serum components increase hexose transport without initiating proliferation.