O2 tensions (Po2) were measured with microelectrodes in multicellular spheroids from EMT6/Ro and V-79-171-B cells. The measurements were performed in spheroids kept in flowing growth medium that was equilibrated with 5% CO2 and air at a temperature of 37 degrees C and contained 5.5 mM glucose. The recorded Po2 profiles are characterized by a diffusion-depleted zone surrounding the spheroids and by a steep drop in Po2 within the spheroids over mean distance of 220 and 188 micrometer from the surface of EMT6/Ro and V-79-171B spheroids over mean distance of 220 and 188 micrometer from the surface of EMT6/Ro and V-79-171B spheroids respectively. Smaller spheroid exhibit parabolic Po2 profiles, larger ones show a central plateau. The region of the steep decrease in Po2 corresponds to the thickness of the viable rim: the plateau region is created by the absence of O2 consumption in the central necrotic area. Po2 in the centre of EMT6/Ro spheroids decreased from 66 mmHg at a diameter of 400 micrometer to 13 mmHg at a diameter of 1000 micrometer. Under the present conditions during growth and in the experiments, values below 5 mmHg were recorded only in spheroids 1200 micrometer. Comparably low Po2 was recorded in V-79 spheroids with diameters of 650 micrometer +. In spheroids of this cell type with a diameter of 400 micrometer, Po2 was 42 mmHg. The findings provide evidence that necrosis may arise at average Po2 of 57 and 42 mmHg in EMT6/Ro and V-79-171B spheroids, respectively, grown under the conditions described.