Many cases have been reported that a post spinal headache can be relieved immediately by an epidural injection of saline; and autologous blood also has recently been used successfully instead of saline. The changes of the cerebrospinal fluid pressure in 40 cases were observed in the present study in support of the concept; that a continuous leakage in association with hypovolemia and hypotension of the cerebrospinal fluid is the primary cause of a post spinal headache. Subarachnoid pressure increased immediately with saline injection into the lumber epidural space. A spinal needle was inserted into the subarachnoid space at the level of L2~3 and opening pressure of the cerebrospinal fluid was read. An epidural Tuohy needle was insertad at the L3~4 and 25m1 of saline was injected into the epidural space and the cerebrospinal fluid pressure was read in the sitting position. Sitting Position Mean pressure after injection 555±(110.9)mm H₂O, Pressure rise rise (%) 51.3%, Mean opening pressure 366±(52.2)mm H₂O, Lateral position: Mean pressure after injection 308±(70.8)mm H₂O, Pressure(%) 86.7%, Mean opening pressure 165±(42.6)mm H₂O. These pressure changes responded almost simultaneously as saline was injected. This pressure rise of 51.3% in the sitting position and 86.7% in the lateral position are clinically very significant. Therefore, it is most possible that the immediate relieve of post, spinal headache by injection of fluid into the epidural space is simultaneous with the increase of the cerebrospinal fluid pressure.