1. The influence of ouabain on net Ca movements was studied in human erythrocyte ghosts by atomic absorption spectrophotometry. 2. Ouabain (10(-5) M) showed a dual effect, altering both entry and exit of Ca from K-rich ghosts incubated in a high-Na medium in the presence of 10 mM-Ca. 3. Stimulation of Ca entry was observed in the first 15 min at 37 degrees C, whereas during the subsequent 15 min incubation ouabain elicited Ca extrusion. This latter effect was eliminated when the ouabain concentration was raised to 1 mM. 4. Ouabain-dependent Ca movements were abolished by replacing both internal K and external Na with choline. They were also absent from ghosts prepared at a high lytic ratio (1 : 100) or obtained from ATP-depleted cells. 5. A moderate increase in cell ATP enhanced the effect of ouabain on Ca efflux whilst it was eliminated at higher ATP levels. 6. The actions of ouabain markedly depended on the initial ADP/ATP ratio in ghosts, being optimal at about 2-5. 7. The results suggest that the effects of ouabain on Ca movements are mediated through the Na pump. Reversal of this pump in Na-rich K-free medium may provide the energy for active Ca transport.