In order to characterize Ca2+-transport in red cells stored in ACD Ca2+-loading and Mg2+-depletion by the ionophore A23187, CaATPase activity determination in intact cells and an accurate Ca2+-influx technique were adapted to preserved blood. Active Ca2+-efflux (pump) was measured in rejuvenated cells loaded by Ca2+ with A23187. The rate of Ca2+-pump declined only slightly during 3 weeks of storage (from 80 ± 15 to 66 ± 17 μmoles Ca2+/l. cells/min) and a marginal trend of decrease in the Ca:ATP ratio was observed (from 1.96 ± 0.15 to 1.88±0.11). Passive Ca2+-influx (leak) was studied in regenerated cells in which the Ca2+-pump was blocked with 0.2 mm lanthanum. Ca2+-influx showed a slow increase during the first 3 weeks of storage (from 0.4±0.16 to 1.25±0.4 μmoles Ca2+/l. cells/min), later it increased rapidly. Passive Ca2+ leak and exchange transport were studied in unregenerated, phosphate ester-depleted cells. In this case passive Ca2+-influx increased 2–3-fold as early as 1–2 d after storage in ACD. This initial increase was followed by a continuous slow enhancement that reached a flux of 3.5±0.7 μmoles Ca2+/l. cells/min after 3 weeks of storage. The passive Ca2+-permeability increase that occurred during storage could be readily compensated by the Ca2+-pump without causing metabolic imbalance. The Ca2+-transport, of unregenerated stored cells, however, showed impairment under certain conditions (A23187+EDTA and lanthanum treatments, ghost preparation). The Ca2+-induced shape changes were reversible and ran parallel with the cell Ca2+ level during Ca2+-pumping up to 5 weeks of storage. This finding indicates a direct relationship between cell Ca2+ and shape.