Glucose is normally required as the energy source and for the proliferation of neoplastic cells. For Ehrlich ascites tumour cells, kept under glucose-free culture conditions, this requirement was alleviated by uridine, indicating that the supply of ribose is obligatory for sustaining growth capacity. In a 96-hr culture experiment with mouse-derived cells, the increase in cell number from cultures supplemented with 5 mM uridine was 50-70%, whilst lactate production was 5% that of controls. An increase in the number of multinucleate cells was observed from cell-smears; DNA histograms indicated the presence of cells with a DNA content higher than 4c and an increased portion of cells in G2 phase. For precise determination of changes in cell cycle distribution on transfer of cells from glucose-supplemented to glucose-free conditions, the progression of phase-accumulated cells (by centrifugal elutriation) was monitored by DNA distribution analysis; G2 cells continued the cycle at a rate comparable to controls but were delayed, in the following cycle, predominantly in S and G2 phases. This was also observed with G1 cells from a G1-accumulated fraction in the first cycle. The addition of glucose to cells kept for some hours in glucose-free, uridine-supplemented medium resulted in an immediate increase in mitotic index (amplification by the colcemid method). The results are interpreted and support our concept that the delivery of compounds, necessary for normal growth, i.e. hexoses for glycoproteins and glycolipids, are limited as a consequence of the 'metabolic channelling' of pentose from uridine in Ehrlich ascites tumour cells. Therefore, the constantly lowered growth-rate in uridine-supplemented cells observed with long-term culture experiments could reflect an adaptation of growth-cycle to these limitations.