Abstract: A method and apparatus for preparing thin layers, such as thin gel layers useful in electrophoresis, wherein a liquid layer material is introduced to the space defined between two parallel mold surfaces on first and second mold elements. Initially, the mold surfaces overlap each other to a small extent, and liquid is introduced to the space between the surfaces adjacent the leading edge of the top mold element. The mold elements are then shifted with respect to each other while maintaining their parallel relationship, and additional liquid layer-forming material is introduced to the space during the shift. In a preferred embodiment, a third mold element is situated adjacent to and spaced a small distance from the bottom mold element, and liquid material is introduced to the space between the bottom mold elements during shifting of the top mold.