Abstract: A transformer having a laminated core and a coil enclosing a leg portion of the core, has a thin plastic frame means interleaved between an adjacent pair of core laminations in liquid-tight relation therewith to form, with the pair of core laminations, a completely enclosed internal passageway in the core for the circulation therethrough of a liquid cooling medium for cooling the core during transformer operation. The frame means has inlet and outlet connections projecting outwardly of the core for connection to a supply of a liquid cooling medium, and they communicate with the opposite ends of the internal cooling liquid passageway in the core through slot-shaped openings extending edgewise through the frame side walls.