Human Glioma U-118 MG and hamster V-79-379A spheroids were used as models of the nodular structure often seen in poorly vascularized regions of solid tumours. K+, Ca2+ activities and membrane potentials were measured in the spheroids, using microelectrodes. Both stable and peak values were obtained. All values were included in the analysis to allow an accurate estimate of the relative number of electro-physiologically active cells at different depths in the spheroids. The relative number of active cells decreased drastically with depth although most cells in the corresponding regions looked morphologically viable. The amplitude values of potentials and K+ activities showed, however, no significant variations with depth. The Ca2+ activity varied largely between individual cells. The relative number of proliferative cells decreased with depth in the spheroids, in parallel to the decrease in the number of electro-physiologically active cells. The results indicate that the fraction of cells, in poorly vascularized regions, having normal metabolism might be fewer than what can be estimated only on a morphological basis. Thus, the amount of cells with the capacity to contribute to growth might be overestimated when inspecting histological sections.