In a heat pipe which is filled with a heat transfer medium at least one flow duct each are provided for the liquid heat transfer medium and for the heat transfer medium to be evaporated. The flow duct (2) for the liquid heat transfer medium has a cross-section whose capillary radius increases continuously in the direction of the condenser-side end of the heat pipe. A sealable opening is preferably provided at the evaporator-side end of the heat pipe in order to interconnect the flow ducts. The shut-off device (16) for this through opening for bubbles of gas or steam which have collected in the liquid duct (2) can be operated from outside and can be designed as an electromagnetic valve. <IMAGE>
