Abstract: An intravascular puncturing nozzle with an automatic no blood reflux mechanism. The mechanism includes a part (3) resiliently biased towards a position closing the passage (6) of the plastic nozzle (2), which position is automatically reached when the needle (4) puncturing the blood vessel is removed from such passage. The connector (5) on the serum equipment automatically opens the passage (6) when the said equipment is connected. The mechanism is applicable to intravascular puncturing nozzles to prevent the patient's blood from refluxing and coming into contact with the operator, thereby avoiding contagion, and in order not to put the sterility of the material used at stake.