The interrelationship among external O2 and glucose supply, oxygenation status, oxygen consumption rates and cellular viability in tumour microregions was studied using the multicellular spheroid model. For chronic exposure to various supply conditions multicellular EMT6/Ro spheroids were cultured in stirred media equilibrated either with 20% (v/v) or 5% (v/v) oxygen and containing four different glucose concentrations ranging from 0.8 mM to 16.5 mM. Spheroids were investigated using histology and O2-sensitive microelectrodes for measuring oxygen tension (PO2) values. A chronic decrease of the glucose concentration in the medium is associated with a substantial reduction in the thickness of the viable rim of cells and with a persistent increase in the cellular respiration rate. In general, both viable rim size and respiration are decreased through restriction of O2 supply during spheroid growth at a given external glucose concentration. The O2 consumption in spheroids appears to decrease with increasing spheroid size under most of the growth conditions investigated. These findings provide evidence for a large capacity of the spheroid cells to chronically adapt their metabolic rates to different supply situations. The experimental data and theoretical considerations indicate that necrosis may develop in the centre of these spheroids due to the lack of O2 and/or glucose under some of the growth conditions, but central necrosis can also occur despite sufficient O2 and glucose supply. Consequently, cellular metabolism and viability in tumour microregions may not be determined by the diffusion limitation of O2 or specific substrates alone, such as glucose, but may be influenced by a complex interaction of factors in the micromilieu the majority of which are still unknown.