Concentration of free cytoplasmic Ca2+ ([Ca2+]i) in Ehrlich ascites tumour cells loaded with fura-2 was measured in single cells applying a video imaging system. In resting cells [Ca2+]i amounted to 60-340 nM and was increased after addition of 10 mM D-glucose or D-2-deoxyglucose by 80-200 nM. This increase occurred within 30-60 s following addition of the sugars and lasted for several minutes. Pretreatment of the cells with thapsigargin resulted in a much smaller [Ca2+]i increase after addition of glucose or deoxyglucose and, vice versa, thapsigargin added after the sugars mobilized less Ca2+ than when added before. A possible relation of the [Ca2+]i rise evoked by glucose and deoxyglucose to the Crabtree effect is discussed.