A thermocycler apparatus and method for rapidly performing the PCR process employs at least two thermoelectric modules which are in substantial spatial opposition with an interior space present between opposing modules. One or multiple sample vessels are placed in between the modules such that the vessels are subjected to temperature cycling by the modules. The sample vessels have a minimal internal dimension that is substantially perpendicular to the modules that facilitates rapid temperature cycling. In embodiments of the invention the sample vessels may be deformable between: a) a shape having a wide mouth to facilitate filling and removing of sample fluids from the vessel, and b) a shape which is thinner for conforming to the sample cavity or interior space between the thermoelectric modules of the thermocycler for more rapid heat transfer.