Continuously proliferating cells exactly double their mass during each cell cycle. Here we have addressed the controversial question of if and how cell size is sensed and regulated. We used erythroblasts that proliferate under the control of a constitutively active oncogene (v-ErbB) or under the control of physiological cytokines (stem cell factor, erythropoietin and v-ErbB inhibitor). The oncogene-driven cells proliferated 1.7 times faster and showed a 1.5-fold increase in cell volume. The two phenotypes could be converted into each other 24 h after altering growth factor signalling. The large cells had a higher rate of protein synthesis, together with a shortened G1 phase. Additional experiments with chicken erythroblasts and mouse fibroblasts, synchronized by centrifugal elutriation, provided further evidence that vertebrate cells can respond to cell size alterations (induced either through different growth factor signalling or DNA synthesis inhibitors) by compensatory shortening of the subsequent G1 phase. Taken together, these data suggest that an active size threshold mechanism exists in G1, which induces adjustment of cell-cycle length in the next cycle, thus ensuring maintenance of a proper balance between growth and proliferation rates in vertebrates.