Effects of ifenprodil tartrate on whole blood filtrability ex vivo in rats was investigated by a standard technique for measuring whole blood filtration time passed through a filter (5 micron). Ifenprodil tartrate was observed to reduce the filtration time dose-responsively. This effect was especially evident at 20 mg/kg (P less than 0.05). On the other hand, no effect on the hematocrit value, plasma concentration of fibrinogen, erythrocyte count or mean cellular volume of erythrocytes was observed. These results indicated the increasing effects ex vivo of ifenprodil tartrate on erythrocyte deformability. The ex vivo effects on erythrocyte deformability was manifested without changing the ATP contents, ATP/ADP ratio or adenylate energy charge in erythrocytes; and a phenothiazine-like amelioration in the shape of crenated erythrocytes was observed. These results suggested that the effect of ifenprodil tartrate on erythrocyte deformability ex vivo might be due to direct action on the erythrocyte membrane. At 0.3 mg/kg, i.v., ifenprodil tartrate significantly (P less than 0.05) increased the blood flow in the hypothalamus of conscious rats. Thus, it was indicated that ifenprodil tartrate, which increases the erythrocyte deformability and the cerebral blood flow, is useful for the therapeutic treatment of cerebrovascular accidents.