Hydrant having a main valve which is intended for installation in a water supply network, having: a) a connector ( 108,28) which serves for the removal of water; b) a riser pipe and an actuable valve rod which extends therein, is connected to the main valve, and by means of which the main valve can be adjusted between a closed position and an open position which is necessitated for the removal of water; c) a valve seat and a closure body which is intended to interact with the valve seat and is a constituent part of the main valve; and d) a draining opening which is provided for automatic emptying of the water which remained in the hydrant in the case of non-use of the hydrant; e) the main valve having means for preventing the introduction of medium through the valve seat into the water supply network via the hydrant in the case of an open position of the main valve for the removal of water and in the case of an absence of a throughflow from the water supply network through the hydrant, characterized in that the means on the main valve comprise: f) a base which is connected fixedly to the valve rod; and g) the closure body which, facing the valve seat, is connected to the base in a manner which is mounted on a spring: h) the closure body being seated with its main seal on the valve seat so as to seal it in the closed position of the main valve, and the base having moved onto the closure body counter to the force of the spring; i) the base having moved upwards in the open position of the main valve and in the case of an absence of a throughflow from the water supply network through the hydrant out of the connector ( 108,28), and the closure body being pressed sealingly with its main seal onto the valve seat by the force of the spring; j) the base remaining in an upwardly moved state in the open position of the main valve and in the case of the presence of a throughflow from the water supply network through the hydrant out of the connector ( 108,28), and the closure body being pressed away with its main seal from the valve seat counter to the force of the spring by the pressure of the throughflow, namely by the differential pressure which occurs in the process in the flow; and k) the base having moved upwards in the open position of the main valve and in the case of an attempted introduction of medium via the hydrant into the water supply network, and the closure body being pressed sealingly with its main seal onto the valve seat by the force of the spring and by the pressure of the prevailing medium, as a result of which the introduction of medium into the water supply network is blocked. a) a connector ( 108,28) which serves for the removal of water; b) a riser pipe and an actuable valve rod which extends therein, is connected to the main valve, and by means of which the main valve can be adjusted between a closed position and an open position which is necessitated for the removal of water; c) a valve seat and a closure body which is intended to interact with the valve seat and is a constituent part of the main valve; and d) a draining opening which is provided for automatic emptying of the water which remained in the hydrant in the case of non-use of the hydrant; e) the main valve having means for preventing the introduction of medium through the valve seat into the water supply network via the hydrant in the case of an open position of the main valve for the removal of water and in the case of an absence of a throughflow from the water supply network through the hydrant, characterized in that the means on the main valve comprise: f) a base which is connected fixedly to the valve rod; and g) the closure body which, facing the valve seat, is connected to the base in a manner which is mounted on a spring: h) the closure body being seated with its main seal on the valve seat so as to seal it in the closed position of the main valve, and the base having moved onto the closure body counter to the force of the spring; i) the base having moved upwards in the open position of the main valve and in the case of an absence of a throughflow from the water supply network through the hydrant out of the connector ( 108,28), and the closure body being pressed sealingly with its main seal onto the valve seat by the force of the spring; j) the base remaining in an upwardly moved state in the open position of the main valve and in the case of the presence of a throughflow from the water supply network through the hydrant out of the connector ( 108,28), and the closure body being pressed away with its main seal from the valve seat counter to the force of the spring by the pressure of the throughflow, namely by the differential pressure which occurs in the process in the flow; and k) the base having moved upwards in the open position of the main valve and in the case of an attempted introduction of medium via the hydrant into the water supply network, and the closure body being pressed sealingly with its main seal onto the valve seat by the force of the spring and by the pressure of the prevailing medium, as a result of which the introduction of medium into the water supply network is blocked.