Abstract: A heater for heating fluid within a body cavity of an animal say for heating tumours by irrigation, comprises a container having inlet and outlet ports through which fluid in the cavity is passed by an external pump, and a heating element in the container to heat at least some of the fluid that passes through the container. Pumping can be achieved by a fluid filled pipe coupled to the container, either directly by or by way of a diaphragm, and to a bellows or like arrangement for cyclicly varying the pressure of the fluid in the pipe and container.