A pressure activated two-way slit valve assembly is designed to be used in combination with, but not limited to, a high flow rate catheter to prevent accidental ingestion of air or loss of blood if the closure cap comes off during non-use of the catheter. In addition, the potential for occlusion of the catheter due to blood clots in the catheter and catheter related infection is substantially reduced. The pressure activated two-way slit valve assembly includes a first end, a second end, a wall defining a dumbbell shaped channel, and a flexible, thin disk having a slit and positioned within the pressure activated two-way slit valve assembly to reside within the dumbbell shaped channel. The slit and the dumbbell shaped channel are sized to enable the slit to deform in response to a predetermined pressure differential across the slit to allow fluid to pass therethrough.