A heat pipe (1) containing a working fluid, comprises an evaporator (3), a condenser (2), an aperture (16) interconnecting the evaporator and the condenser, and a regulating means (13, 14) for limiting the maximum temperature in the condenser to a predetermined temperature. The regulating means comprises a plug (13) operable by means of a temperature sensitive member (14) to move between a first position in which the aperture is open and a second position in which the plug covers the aperture to restrict the flow of working fluid from the condenser to the evaporator. The working fluid then collects in the condenser and acts on the plug to further restrict the flow of working fluid from the condenser to the evaporator.