Abstract: A water activated therapeutic moist heat pad structure is described formed with outer porous water retaining layers which enclose an inner layer of distributed heat generating heat cells and an associated water retaining absorbent inner porous layer. In a preferred embodiment the heat cells are formed of electrochemical heat cells wherein a suitable dry electrolyte is provided such as table salt. The inner porous layer is sized to provide the electrochemical heat cells with adequate electrolyte to use up their active ingredients and achieve an extended heating period. The segmentation of the heat cells and their distribution throughout the structure combine to provide a flexible therapeutic moist heating pad which can conform to the body surface to be treated.