1. In inside-out vesicles of high potassium permeability, prepared from human red cell membranes, volume changes accompanying the action of the Ca2+ pump were measured by recording the intensity of light scattered by a suspension of these vesicles. Replacing Cl- by the impermeant gluconate anion changed swelling into shrinking. 2. Assuming that in Cl- media two Cl- ions accompany one Ca+ ion moved by the pump and in gluconate media two K+ ions are exchanged for one Ca2+ ion resulted in a good agreement between relative Ca2+ transport rate obtained from the volume change and from direct measurement of 45Ca uptake in the two media. 3. The fact that it is possible to change co-transport of Ca with Cl- into counter-transport of Ca2+ for K+ rules out that within the pump there is an obligatory coupling of Ca2+ movement with movement of another ion species (including the proton). The conclusion, therefore, is that the Ca2+ pump must be electrogenic. 4. The combination of measurement of volume change with direct measurement of 45Ca movement yielded 5-6 microliter/mg protein for the volume of the vesicles.