Oxygen consumption, glucose, lactate, and ATP concentrations, as well as glucose and lactate turnover rates, have been studied in a three-dimensional carcinogenesis model of differently transformed rat embryo fibroblasts (spontaneously immortalized Rat1 and myc-transfected M1, and the ras-transfected, tumorigenic descendants Rat1-T1 and MR1) to determine metabolic alterations that accompany tumorigenic conversion. Various bioluminescence techniques, thymidine labeling, measurement of PO₂ distributions with microelectrodes, and determination of cellular oxygen uptake rates (c_O2) have been applied. In the ras-transfected, tumorigenic spheroid types, the size dependencies of some of the measured parameters exhibited sharp breaks at diameters of ~830 µm for Rat1-T1 and ~970 µm for MR1 spheroids, respectively, suggesting that some fundamental change in cell metabolism occurred at these characteristic diameters (denoted as "metabolic switch"). c_O2 decreased and lactate concentration increased as functions of size below the characteristic diameters. Concomitantly, glucose and lactate turnover rates decreased in MR1 spheroids and increased in Rat1-T1. Spheroids larger than the characteristic diameters (exhibiting cell quiescence and lactate accumulation) showed an enhancement of c_O2 with size. Systematic variations in the ATP and glucose levels in the viable cell rim were observed for Rat1-T1 spheroids only. Proliferative activity, c_O2, and ATP levels in small, nontumorigenic Rat1 and M1 aggregates did not differ systematically from those recorded in the largest spheroids of the corresponding ras transfectants. Unexpectedly, respiratory activity was present not only in viable but also in the morphologically disintegrated core regions of M1 aggregates. Our data suggest that myc but not ras transfection exerts major impacts on cell metabolism. Moreover, some kind of switch has been detected that triggers profound readjustment of tumor cell metabolism when proliferative activity begins to stagnate, and that is likely to initiate some other, yet unidentified energy-consuming process.