Dantrolene sodium in vitro inhibited the ATP-dependent efflux of calcium from human Fed cells, the Ca⁺⁺-ATPase activity of red blood cell membrane fragments (RBCMF) and passive calcium binding to RBCMF. These effects were obtained With concentrations of dantrolene sodium between 2.5 and 20 uM. However the passive influx of Ca⁺⁺ was measured at 37℃ in cells pretreated to abolish Ca⁺⁺ pumping and was not influenced by dantrolene sodium. From these results, it was concluded that dantrolene sodium inhibits an active Ca⁺⁺ extrusion across the red cell membrane by inhibiting Ca⁺⁺-ATPase activity which is intimately involved with the Ca⁺⁺ transport mechanism in the red cell membrane.